Visitor from interstellar space

Visitor from interstellar space

By now I hope you’re heard about the interstellar interloper that’s been passing through the inner solar system recently. This asteroid, which has been named ‘Oumuamua, is the first-ever discovered object that has been observed coming into the solar system from elsewhere in the Milky Way that is larger than tiny bits of dust.

There are several known instances of objects being “ejected” out of our solar system, so periodically there should be a chance to see a passing object that has been “ejected” from some other solar system passing by us. But only objects that pass close to the Sun or those that cast their own light are bright enough to be seen.

‘Oumuamua (NASA artist’s impression)

‘Oumuamua was discovered on Oct. 19th when it was already more than a month past its closest point to the Sun. It’s only about 300 feet across, and it was close enough to Earth to be seen for a short while.

Objects passing through the solar system that aren’t gravitationally bound to the Sun must be moving very quickly, although all paths are bent around their closest point to the Sun. The eccentricity of an elliptical orbit around the Sun is represented in the mathematical elements of the orbit by a number between zero and 1. The eccentricity of an object that is going too fast to be captured by the Sun has a value between 1 and infinity. The value in this case is about 1.2, and the object’s velocity entering the solar system was about 59,000 mph. It was probably ejected from another star many light years away and millions of years ago, although it is from the “disk” of the Milky Way and not the more exotic “halo”. A “halo” object would probably be moving faster.

‘Oumuamua’s orbit can be simulated in Starry Night Dome. The orbital elements are a little on the uncertain side because of the short duration of observations between discovery and it zipping out of range. And the effect of the over-unity eccentricity appears to “break” the position of the object during times before February 2016 or after March 2019. But the 3 years when it’s at its closest point to the Sun are replicated pretty well.

To add ‘Oumuamua to SciDome Version 7, right-click on the Sun in Starry Night Preflight and select ‘New Asteroid…’ and enter the following values in the details window that pops up, using the ‘Pericentric’ method instead of ‘Near-Circular.’ Also pick an appropriate name in the ‘Untitled’ field.

E=1.1993471
Q=0.2552817
Node=24.5994750
W=241.6917568
I=122.6813370
Tp=2458005.98918377
Epoch=2458056.5

Once you close this details window and “keep” the new object, and quit SciDome properly, the new object will be written to a file called “User Planets.ssd”. You need to copy this file from its location on Preflight to the Renderbox for it to be “live” on both computers.

c:\Users\Spitz\AppData\Local\Simulation Curriculum\Starry Night Prefs\Preflight\User Planets.ssd

This file needs to be copied and installed on the Renderbox at the comparable folder location:

c:\Users\Spitz\AppData\Local\Simulation Curriculum\Starry Night Prefs\Renderbox\User Planets.ssd

Please contact me if you need a little extra guidance on making this work. After this is done, the object should be “live” in Starry Night on the dome during the current “Now”.

You can also fly out to the object and watch the planets and the Sun fly by as its lumpy asteroid shape zooms past. I would recommend one special piece of orientation for objects like this. During SciDome training, one of the choices we emphasize when looking at a solar system object from above is that you can “Rotate With” or “Hover Over” the planet or moon below. If you “Rotate With” while looking down at the United States, and speed up time, the Earth won’t appear to rotate, you can see successive nightfalls and daybreaks over North America, and the background stars will rotate around in the background. If you “Hover Over”, as time passes North America will rotate away to the east, Asia will appear out of the west and the fixed stars won’t move. The Sun Angle won’t change much either.

There is a third option in the dropdown menu that allows you to choose “Rotate With” or “Hover Over”, which is very much like “Hover Over” but not quite. It’s called “Follow in Orbit.” The small difference between “Hover Over” and “Follow in Orbit” is that “Follow in Orbit” will maintain the phase of illumination by the Sun as the planet orbits the Sun, and the fixed stars will slowly sweep by although the Sun Angle won’t change as time passes. “Hover Over” is completely inertial – as the planet orbits the Sun, the phase of the Sun Angle will slowly change and only the fixed stars will stay fixed.

We don’t always drill down far enough to distinguish between “Hover Over” and “Follow in Orbit” because it takes more than a month of time flow to accumulate a 30° difference between the two. But in the case of ‘Oumuamua, because its position with respect to the Sun changes so quickly, you might want a way to keep the Sun Angle constant so the source of illumination won’t rotate away from your point of view and you “lose the light”. “Follow in Orbit” is a useful orientation choice for an object like this.

A little bit of hay has been made of the way the incoming path of ‘Oumuamua leads back towards the constellation Lyra. This can also be simulated in Starry Night. The brightest star in Lyra is Vega, which was the fictional location of the first extraterrestrial signals in the Carl Sagan novel Contact. Sagan may have picked Vega to use in his book because it has been known for some time that the motion of the Sun and the solar system through the Milky Way is in the general direction of that star.

The great American/Canadian astronomer Simon Newcomb wrote in Elements of Astronomy – a book Sagan would have known – “The motion of our solar system toward the constellation Lyra is one of the most wonderful conclusions of modern astronomy.” However, as we move in the direction of that constellation, Lyra and the other stars in it have their own movements that will scramble them all out in other directions as time passes.

The proper motions of the stars can also be turned on in Starry Night as a series of artificial lines, and simulated back and forth through a couple of hundred thousand years of movement centered on the present if you switch to a “Stationary Location.” These proper motions may appear random, but if you highlight some of the closest and most well-known nearby stars, you can track them moving away from or toward the direction of Lyra as time passes forward or backward with some coherence.

Gravity Waves From Neutron Star Collision

Gravity Waves From Neutron Star Collision

On August 17th, the Laser Interferometer Gravitational-Wave Observatory, or “LIGO”, detected gravity waves produced by the merger of two neutron stars. There are lots of good takes on this story, and it’s important that we keep retelling it in interesting ways. This entry briefly summarizes the observations of the event, while focusing in on a couple of ways that a SciDome planetarium show about this could be exciting for an educational audience.

Basic components of a gravitational wave detector

A neutron star is the typical remnant left behind after a supernova explosion. Specifically, this is the kind of supernova that overtakes a massive star at the end of its lifespan, when heavy elements have accumulated at its core. Massive stars that go supernova tend to have short lifespans on the cosmic scale, only a few million years.

In a supernova, the outer layers of the star collapse onto the core, compress it, and rebound outwards in an explosion. The core is left behind, but the pressure of the infalling layers squeeze it to the point that normal matter made up of atoms can no longer exist. It is squeezed so tight that orbiting electrons and core protons violate the Pauli Exclusion Principle and combine into neutrons that have no net charge.

A neutron star is a small but extremely massive object made up of this “neutron-degenerate matter” that has no protons or electrons, with no space in between the nuclei. “Neutronium” is extremely dense, so that a teaspoon full of it would weigh two billion tonnes. The diameter of the neutron star is only a few miles.

The type of supernova that can create a neutron star tends to occur in a given galaxy roughly once per century. Neutron stars that rotate can be observed in radio waves as “pulsars”. Due to the law conservation of angular momentum on a rapidly collapsing massive object, pulsars tend to rotate extremely quickly.

The Spitz Fulldome Curriculum, included with every SciDome system, includes dozens of lessons prepared by noted astronomy educator Dr. David Bradstreet

You can review the lesson plan for the Fulldome Curriculum minilesson in your SciDome named “Galactocentric Distributions”, which has a step that highlights the locations of supernova remnants like pulsars. That ATM-4 lesson uses the Starry Night database “Supernova Remnants” which can also be toggled on and off manually.

Although neutron stars are rare (one such may be created in a galaxy of 100 billion stars after an interval of 100 years), it is possible for two pulsars to be observed orbiting each other. Since they were first observed in 1967, hundreds of pulsars have been observed in the Milky Way, and there is also a suspected population of neutron stars that can’t be observed. So far there are two known binary pulsars in the Milky Way.

Until LIGO was completed in 2002 and then upgraded in 2015, the only method of observing the effects of gravitational waves was to use a radio telescope to time the regular pulses of a binary pulsar and measure its rate of slowing. Einstein’s general theory of relativity describes gravity as a warping of space/time around massive objects. As a pair of pulsars orbit each other, some of their rotational energy is carried away by the propagation of gravitational waves. The first known binary pulsar has an orbital period of about 7 hours, and the timing of its pulses has revealed a small orbital contraction. That pair may take 300 million years to spin together.

Since LIGO was upgraded in 2015, it has started to discover gravitational wave events caused by black holes merging. Mergers of stellar-mass black holes are somewhat common in the deep cores of galaxies and maybe some other places in the universe, but so far no black hole mergers have been observed by LIGO with enough accuracy to find their host galaxies.

Worldwide Gravitational Wave Observatories – Caltech/MIT/LIGO Lab [Public domain], via Wikimedia Commons

The two stations of LIGO are located in eastern Washington state and in rural Louisiana. The sensitivity of the system has recently been improved through collaboration with the European gravitational wave observatory Virgo, located outside Pisa, Italy. These locations can be visited in SciDome with The Layered Earth.

The August 17th event that’s now in the news (GW170817) was the collision of a pair of neutron stars in another galaxy. It is notable because it’s the first observed neutron star collision, and because its optical component was found thanks to triangulation between gravitational wave observatories and gamma-ray observatories.

The Swift Gamma-Ray Burst spacecraft is one of the stars of the fulldome show Black Holes: The Other Side of Infinity. Networking of several observatories is also a feature of We Are Astronomers and other planetarium shows.

One aspect of the breaking nature of the story is that although the news was embargoed from distribution until this week, rumors have been circulating about it since the day it happened, a few days before the total solar eclipse in August. Now that the papers have been published, notably B.P. Abbott et al. in Physical Review Letters, the scale of the collaboration becomes apparent. There are so many co-authors on these papers that it has been guesstimated that 15% of all professional astronomers are being cited. No wonder it was difficult for them to keep a secret!

The rapid outburst and the quick dimming of the object was less impressive than a supernova – maybe only a tenth as bright as a supernova, so the name “kilonova” has been proposed to distinguish this kind of an event from a supernova.

https://www.youtube.com/watch?v=nziW8fywwmg
ESO animation: Zooming in on the NGC 4993 kilonova

The host galaxy is also very close to us. One of my other hats apart from Spitz is as a supernova hunter. I’ve co-discovered ten supernovae by searching through images of external galaxies, and although it is not atypical for one or even two supernovae per year to turn up in galaxies that are less than ten million light years away, I have never found one that was closer than 215 million light years. The host galaxy, NGC 4993, is only about 120 million light years away.

NGC 4993 is so close that it was observed visually in the late 18th Century, before photography was invented, with an 18-inch reflecting telescope with a speculum metal mirror. The greatest father-and-son team of astronomers in history – William and John Herschel – found it 45 years apart. 

Did I mention that neutron star collisions are rare? A given galaxy may only host a binary neutron star collision once per 100,000 years, and there are only on the order of 100,000 galaxies in the volume with radius 120 million light years. 

You can look up NGC 4993 in Starry Night and dial up the location and the date of its discovery: Slough, England on March 26th, 1789. This galaxy isn’t easy to see from so far north. It only gets 16° above the southern horizon as seen from Slough. 

You can also dial up the discovery date of the gravitational wave event – August 17th of this year. One of the points that’s been made is that in August, NGC 4993 sets a couple of hours after the Sun, and is not easy to find in the short time available after sunset. By the time of this week’s announcement, NGC 4993 has gone behind the Sun and no new observations can be made by anybody until it returns in the predawn sky. If the gravitational waves event had happened two months later, the source of GW170817 would not have been found.

You can also fly out to the host galaxy to observe it in three dimensions with Starry Night. The name NGC 4993 is not “flyable”, so we have to type in the name that is attached to this galaxy in one of the newer catalogs that renders it as a 3D object. In the search field, type in:

2MASS-13094770-2323017
Origin of Solar System Elements by Dr. Jennifer Johnson

The phrase “We are star stuff” has been a part of the astronomer’s public outreach lexicon for many years, since Carl Sagan. We know how to point out that all the heavy elements that make up solid matter on Earth were fused out of lighter elements in the cores of stars, and that iron and everything heavier than it was produced by supernova explosions. It’s now time to re-invent the part of this expression that deals with a lot of those heavy elements.

The neutron star collision in NGC 4993 seems to have produced more than 10 Earth masses of gold, platinum and other precious metals. Dr. Jennifer Johnson from The Ohio State University, a SciDome user, posted a Periodic Table of the Elements on her blog that has been broken down to highlight which elements can be generated by supernovae, colliding neutron stars, etc.

Simulating Near-Earth Asteroid 2012 TC4

Simulating Near-Earth Asteroid 2012 TC4

For the last couple of days, there has been some news coverage of another small asteroid that’s going to fly close to the Earth tonight. This happens fairly often, although it is a little unsettling when it does. We have just passed the 9th anniversary of the discovery of a small asteroid that was on a collision course with Earth in October 2008. That object produced a fresh field of meteorites in Sudan.

NASA illustration of 2012 TC4 near approach

The new asteroid 2012 TC4, was discovered on October 4th, 2012. It flew past Earth on October 12th of that year, 96,000km away. It’s only 13 meters across, and small asteroids like it and the other one tend to be discovered only when they are close enough to the Earth to be visible in large telescopes. Because the orbit of 2012 TC4 is tangent to the orbit of the Earth in October, the only time during the year when Earth and TC4 can be close together is in October. Since that 2012 encounter, the period of TC4’s orbit has been 1.67 years, so we are bound to be close together again after a five-year interval.

After this evening’s encounter, when only 43,000km will separate Earth and TC4, the period of its orbit will be lengthened to 2.06 years. The asteroid orbit will still be tangent to Earth’s orbit in October, and that extra 0.06 of a year will accumulate and may put the asteroid back close to Earth in October 2033 or 2050.

The way Starry Night simulates asteroid orbits in a conventional way depends on those orbits not changing very much. The Keplerian orbital elements model just requires six numbers that describe the orbit of an asteroid around its parent body. Starry Night models these numbers with an accuracy of up to 7 decimal places, and that’s accurate enough to describe asteroids in most of interplanetary space. Keplerian orbits do not simulate the way the Earth’s gravity can deflect the orbit of an asteroid around the Sun, and any asteroid that passes really close to the Earth will be deflected in that way. So the best way for us to simulate TC4’s encounter with Earth in SciDome is unconventional.

Simulating TC4 on SciDome

I have prepared a “Space Missions” file, composed of a set of state vectors from the JPL website that describe TC4’s path for the 8-day period centered on this week’s encounter. It accurately models the way TC4 will sneak as close to Earth as the belt of geosynchronous satellites, and the orbit should be accurate to 0.1km and about 10 seconds in time.

The space missions dataset, and the JPL data format that can make data for it, are originally meant to simulate spacecraft, not asteroids, but the way the data is presented is mostly the same. Although if you “fly to” TC4 in Starry Night, it will look like a space probe and not an asteroid.

If you have a recently updated SciDome, you can get 2012 TC4 on your dome by downloading this zip file, opening it up, then installing “2012 TC4.xyz” on your Preflight computer at the following folder location:

C:\Program Data\Simulation Curriculum\Sky Data\Space Missions

It may be necessary to create a “Space Missions” folder in Sky Data here. If it is, it should be named just so.

If you have an older SciDome, the destination folder for the new file is a little different. Please contact me for directions.

With that done, the next time you run SciDome V7, you ought to be able to find 2012 TC4 by typing it into the search engine field at the top of the pane you choose to use as your “Find” pane. The “Celestial Path” or “Local Path” can be highlighted to show its course across Earth skies, and its “Mission Path” can be highlighted to represent its three-dimensional path through space around the Earth. The position of TC4 will only be simulated during the current 8-day period. If interest in it persists, its new orbit ought to be stable enough to represent with Keplerian elements after everything settles down.

TC4 will be passing through the constellations Aquarius, Capricornus and Sagittarius this evening. I used the JPL data to set up a prediction and make a reservation to use a 150mm online telescope in New Mexico tonight to try and take some images of TC4 as it passes by. We’ll see how it goes. Please feel free to contact me if you would like a little more guidance on installing 2012 TC4 on your SciDome.

The Total Solar Eclipse of 2017 in SciDome

The Total Solar Eclipse of 2017 in SciDome

Total Solar Eclipse

The appearance of a total solar eclipse is so singular that it is impossible to appreciate it without having seen it for oneself. Unexpected eclipses have changes history in the past, although they can now be predicted. Even having an eclipse forecast in hand does not reduce the impact of the experience on witnesses.

And total eclipses are rare. The most recent time the moon’s shadow fell properly across the United States was in February 1979.

But just as eclipses can be predicted, they can also be simulated in the SciDome planetarium environment. Here is how to simulate the upcoming Great American Eclipse of August 21st, 2017 in your dome.

First you need to set the correct date in Starry Night Dome. Starting from the home location of your planetarium, if you are located in the United States, some time on August 21st the Sun will be at least partially eclipsed, but it probably won’t be totally blocked out.

An accurate simulation of a partial eclipse in SciDome without any magnification will probably look like a normal day as the Sun moves from the eastern to the western horizon across the blue sky and sets. Likewise, in the real world it is possible to not notice the progress of a partial solar eclipse unless the fraction of Sun covered is well over 50%. The shadows cast on the ground by sunlight shining through tree-leaves can reveal a partial solar eclipse, but the reduced amount of sunlight in the sky is only subtly perceptible.

Two steps need to be taken to appreciate a partial solar eclipse in SciDome. First, use the keystroke [Ctrl-D] to toggle daylight off. This will improve the contrast between the Sun and the black sky background.

If you are close to the zone of totality, looking at a hemispheric view, the blown-up image of the Moon we normally use in SciDome to show its phases will probably appear to cover the Sun totally during the partial phases, and toggling daylight back on again with the same keystroke will show that the remaining sunlight is still illuminating the sky above your location. Therefore please right-click on the Sun and select “Magnify”. This will center the Sun on the front of the dome and magnify it so that it’s zoomed in. In this way the “Enlarged Moon” effect used at wide fields of view is eliminated. Now the “bite” out of the Sun is properly simulated. Run time forward and back in steps of up to 300x actual speed to follow the event from start to finish.

To simulate the total phase of this solar eclipse, identify an observing site that will be in the zone of totality. Because the path will neatly bisect the United States, the nearest place where the eclipse will be total is never more than three states away from your location (excepting New England.)

Path of the 2017 Total Solar Eclipse

The following state capitals will be in the zone of totality:

  • Salem OR
  • Lincoln NE
  • Jefferson City MO
  • Nashville TN
  • Columbia SC

Major cities including Kansas City, St. Louis, and Charleston will also be in the zone. You can find alternate locations with the interactive Google Eclipse Map.

Hit the “Viewing Locations” button and enter the name of the town, or its latitude and longitude, or its ZIP code. Once Starry Night has “flown to” your destination, the northern horizon will probably be on the front of your dome, but you can still right-click and “Center” the field of view on the Sun without magnification, and then step forward and back in time in intervals of hours or minutes to find the moment when the sky starts to incrementally darken. The keystroke to advance in 1-hour steps is “h”, and to reverse in 1-hour steps is [Shift-h]. To advance in 1-minute steps, use the “t” key, and to reverse in 1-minute steps, just add the [Shift] key.

Starry Night will not appear to display other than a normal daylight sky until about 8 minutes before totality. During those 8 minutes the sky will start to appear darker and more violet, and extra objects will start to appear in the sky: Venus first, followed by the brighter stars. Then the darkening will accelerate and go into totality for up to 160 seconds of simulated time. The horizon will accurately represent the eerie appearance of the “360° Sunset”, and the occulting Moon will reveal the red solar chromosphere and the white corona surrounding it. After totality is finished, the appearance of the unmagnified sky will take about 8 minutes to return to normal. The partial phases and totality can be simulated accurately with magnification.

Shadow cones, a Starry Night feature to illustrate eclipses

Because SciDome can also accurately simulate travel through space, right-click on the Sun during the eclipse and select “Go there”. Without specifying a surface location, Starry Night will arrive at a point somewhere above the Sun’s surface, with no Earth or Moon in sight. Type in “Earth” in the Find Pane’s search engine field, and then you can right-click on it and select “Magnify”. The disk of the Earth should appear centered on the dome with the far side of the Moon superimposed on it, with the Moon’s shadow being cast on the United States below. By right-clicking on the Moon and selecting “Shadow Cones”, the dimensions of the shadow falling on the Earth will be demonstrated. Step forward and back in time with “t” to see the shadow move across the disk of the Earth.

Right-click on the Moon again and select “Orbit” to show the Moon’s orbital path falling in front of and behind the Earth from this observing location near the Sun. Zoom out a couple of degrees to see the whole arc of the Moon’s orbit to the left and right of the Earth. The keystroke “m” and [Shift-m] can be used to step forward and back by intervals of months, demonstrating that the Moon’s orbit is slightly tilted such that it rarely creates the perfect syzygy between the Earth and Sun to create a solar eclipse.

Great Comet of 1811 (1812?)

Great Comet of 1811 (1812?)

Great Comet of 1811 as drawn by William Henry Smyth

The May issue of Sky and Telescope magazine has a timely item about “Napoleon’s Comets”. The most important of these was the Great Comet of 1811, which was the brightest comet with the longest duration of brightness on record (260 days) until Comet Hale-Bopp shattered that record in 1997.

It is referred to as “Napoleon’s Comet” because of the Napoleonic Wars and the impending War of 1812, in which the United States was allied with France, Germany and Austria against Britain, Spain, Portugal, and Russia. The wars are the backdrop for the novel War and Peace by Tolstoy, and also the newly Tony-award-nominated Broadway musical Natasha, Pierre and the Great Comet of 1812 based on a small part of the novel.

The Comet of 1811 was discovered in March of that year in what is now the constellation Puppis, and it was very bright in the evening sky in September and lingered for the rest of that year. The head and coma of the comet was reported to be wider than the diameter of the Sun and it had a very long, bright tail despite not coming very close to the Earth. The Comet was held to be responsible for unusually fine vintages of French wines harvested from the Autumn 1811 grape harvest, and it is possible that Napoleon was influenced in his decision to invade Russia in June 1812 if he thought of the comet as a portent of victory.

In the US midwest, the Comet was visible during the New Madrid Earthquakes in December 1811. The Shawnee leader Tecumseh, who was born in the year of the Comet of 1769 and was named accordingly, invoked the Comet of 1811 as he built a confederacy of tribes which allied with the British in the War of 1812.

The Comet of 1811 is only mentioned on one page at the conclusion of the first half of War and Peace, but it’s misnamed the Comet of 1812. Accordingly, although the musical is titled Natasha, Pierre & the Great Comet of 1812, the Comet only appears in the finalé and is not depicted in the publicity for the production. You have to go and see it for yourself. Dave Malloy, the creator of the show, says the Comet nevertheless got into the title of the show “for cosmic epicness”.

The Broadway production this year has been nominated for 12 Tony awards, so I can’t imagine it not being talked about in planetariums.

SciDome Implementation

You can add the orbit of the Great Comet of 1811 to your SciDome by right-clicking on the Sun in Starry Night Dome Preflight and selecting “New Comet…” In the orbit specification window that pops up, enter the following values:

Name: Great Comet of 1811
Eccentricity: 0.9951250
Pericentre distance: 1.0354120
Ascending node: 143.0497000
Arg of pericentre: 65.4097000
Inclination: 106.9342000
Pericentre time: 2382768.2562000
Elements epoch: 2382760.5

And in the “Other Settings” tab, change the Diameter to 40 km and change the Absolute magnitude to 0.
“X” out of the new orbit window and confirm you want the changes to be saved. Then quit out of Starry Night properly.

If you are using Starry Night Dome version 6, the comet will be loaded on to the Renderbox when Starry Night is properly exited and will be available the next time the application is started. Because it is a user-created object, though, it will be automatically “hidden” until you uncheck it in the “Hide” column of the Find Pane. Then you can save some favourites showing the sky in the year 1811 featuring it for later playback.

If you are using Starry Night Dome 7, the comet will be saved into a file named User Planets.ssd in the Preflight folder:

C:\Users\Spitz\AppData\Local\Simulation Curriculum\Starry Night Prefs\Preflight

And that file will need to be manually ported over to the corresponding location on the Renderbox. Future versions of Starry Night Dome V7 will make this process automatic.

 

Ben Franklin’s Birthday and the Gregorian Calendar

Ben Franklin’s Birthday and the Gregorian Calendar

Benjamin Franklin

Today is Benjamin Franklin‘s birthday under the calendar we use today, although he was born on the 6th of January of 1706. He was born before the Gregorian calendar reform was implemented in the English-speaking world.

The Gregorian calendar reform adjusted the way that leap years are counted. Instead of observing an intercalary day in February once every four years, the Gregorian observes one such day every four years except when the year is divisible by 100, except when the year is also divisible by 400.

The exact time it takes the Earth to go around the Sun 365.2422 days, not an integer number of days. During the Julian period the remainder was reduced from the 365-day year by adding a leap day every four years. However, the remaining error compounded. The Gregorian calendar uses 97 leap days every 400 years instead of 100 leap days, so the average length of the Gregorian year is 365.2425 days. The Gregorian change also ran a correction to delete the accumulated lag, which had grown to 10 or 11 days.

The new calendar came into force in Roman Catholic states in 1582.

Denmark switched to the Gregorian calendar in mid-February 1700.

The British Empire made the change in 1752.

For any celebrity birthdates you want to celebrate that are older than a certain limit and come from a certain country, it may be important to see if they should be read out as a Julian or a Gregorian date.

If you bring up Starry Night with a date of October 4th, 1582, and advance by one day, you can observe the 10-day correction when the Gregorian calendar was assumed.

The only alternative to observing the ten-day gap that followed in 1582 when looking at earlier dates is to use the proleptic Gregorian calendar, which eliminates the need for a Julian calendar correction when observing past dates. I wouldn’t recommend using the proleptic Gregorian calendar for earlier dates because the people of the time did not use it either, and Starry Night will read out those dates using the Julian.

By 1752, when the British Empire adopted the Gregorian, the accumulated error had grown to 11 days, and the change was reflected in the British colonies that later became the United States. Benjamin Franklin had already been publishing Poor Richard’s Almanac since 1733, and he included a long explanation of the calendar reform in the 1752 edition.

(When Abraham Lincoln used an almanac to show the phase of the moon during the Trial by Moonlight in 1858, as in our Fulldome Curriculum, he was taking a page from one of the most popular kinds of document in the English language other than the Bible.)

The calendar reform of 1752 didn’t catch everyone by surprise, and although the correction was run in September 1752, Franklin had adequate notice before his publication deadline the previous year. The British Parliament passed the new rule as the Calendar (New Style) Act 1750, although the code used for that legislation was “24 Geo. 2 c.XXIII”, meaning it was the 23rd piece of legislation that received royal assent in the 24th year of the reign of King George II. King George had commenced his reign in 1727.

The first point of the new law, before the Julian correction, was to correct the date of the beginning of the legal new year. Although different cultures have strong traditions to begin the new year on January 1st, even now it is impractical for all of our traditions to line up on a single start date: the school year and the NFL season being a couple of examples. The British Empire up to 1752 had observed the start of its legal year on March 25th. The law passed in 1751 corrected the New Year to January 1st at the beginning of 1752, so the official year 1751 was only 282 days long.

Starry Night does not incorporate any of the other weirdness around calendar reform, except that the new year always starts on January 1st, and there is a year Zero in between the BC and CE periods. (ATM-4 does not calculate a Year Zero).

Happy Birthday to Ben Franklin, who was not born on Blue Monday (It was a Sunday in both calendars, and the days of the week have never accumulated an error).