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 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

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).

Equinox: The Big Dipper Star Clock

The pointer stars Merak and Dubhe point at Polaris and go once around per sidereal day, every 23 hours 56 minutes and 4 seconds.  By making a daily correction it is possible to estimate the time at night using these three stars.

One point on this is pretty much intuitive. What is the Right Ascension of Merak? Well, it’s about the same as Dubhe, because a straight line joins both stars to the axis of the celestial clock, the north celestial pole.

But what is that angle, really? Well, it’s 11 hours of Right Ascension. Or, if you use Daylight Saving Time, it appears to be 12 hours.

Thanks to “reasons”, we now spend at least 7 months of the year on Daylight Saving Time, and this helps to make the Big Dipper Star Clock easier to use. At midnight Eastern Daylight Time on September 21st (most years, September 22nd this year), the Autumnal Equinox, the line between Polaris, Merak and Dubhe points straight down to the northern horizon. At midnight Eastern Daylight Time on March 21st, the Vernal Equinox, the line between Polaris, Merak and Dubhe points straight up to the zenith.

On another day of the year, at the moment when the line joining Merak, Dubhe and Polaris is vertical in either direction, if you know the date, you can make an estimate of the time in Daylight Saving Time.

  1. Count the number of days before or since September 21 or March 21
  2. For each day before September 21st, imagine winding your mental clock forward from midnight by 4 minutes. If after September 21st, imagine winding your clock back 4 minutes per day. If your index date is March 21st, use the same directions.

So because tonight is 0 days after September 21st, at the moment tonight when the line between Polaris, Merak and Dubhe points straight down to the northern horizon, you can estimate that it’s about 0*4 minutes before midnight, Eastern Daylight Time. (copy and paste, and replace the zeroes for different dates.)

This system doesn’t work perfectly because it’s the same Daylight Saving Time in Detroit as it is in Boston, although the angles of objects in the sky are different. But it is predictable, if you know that you live east or west of the Central Meridian of your Time Zone, to expect objects in the sky to run reliably fast or slow.

Please refer to the Bradstreet Fulldome Curriculum Vol. 2 Minilesson ‘Time Zones’ and the printed lesson plan in the binder for more detail on time zones. The asterism line representing the Pointer Stars, and the Meridian to show the time when the Pointer Stars are vertical, can be drawn in with Starry Night, and in ATM-4 as well. Please refer to the FDC 2 ‘Hour Angles’ Minilesson and its printed lesson plan to interpret how to use other objects in the sky as a star clock. Because the Pointer Stars are vertical at (DST) midnight on the Equinoxes, they are terrifically useful for this, but the Minilesson might suggest stars for you that have hour angles that are also convenient to remember for relating to the dates that index our seasons. Right-click on a star and select ‘Hour Angle Lines’ to get a dynamic readout of the time since the last moment it crossed the local meridian to the south. The minilessons ‘Time LAST (Local Apparent Solar Time)’ and ‘Time LMST (Local Mean Solar Time)’ use very similar tools.