Watching an Eclipse – FROM THE MOON!

   

This is our second look at the total solar eclipse of July 2, 2019. This time, we will watch it from the Moon, because that is a good place from which to get an overall view of how a solar eclipse works.

We will start with a series of short videos showing how to set up to watch the eclipse. If you watch those, treat them as a tutorial and follow along in another window. It will be easier to do that if you have worked throught the Space Travel Tutorial. We assume that you at least know how to enter the Planetarium, the first segment of the Stargazing Tutorial.

The last video is standalone. In it, we just watch the eclipse, with some commentary. If that is what you want to do, then just skip to that one.

1. Get onto the Moon.

  1. Open the Control Panel by right clicking the mouse.
  2. Set Space Travel mode.
  3. Set your Home Planet to the Moon.
  4. Leave the Control Panel open.
2. Set up date and time.
  1. Open Date and Time on the Control Panel by left-clicking on that heading.
  2. Press Stop Clock, so the time doesn't run away from us while we are setting things up.
  3. Set the timezone offset to 0 so we are all using UTC + 0, which is GMT.
  4. Set the date to July 2, 2019 (2019/7/2) and set the time to 16:30 hours.
  5. Set the Time Rate to 60. That way each hour of Planetarium time will take only one minute of real time to go by.
3. Set up your view and your location on the Moon.
  1. On the Control Panel, open View by left-clicking on that heading.
  2. Select Earth under Tracking and under View Planet.
  3. Close View by left-clicking on the heading again.
  4. Open Viewer Location.
  5. Set Latitude to about 0.
  6. If you need to, change Longitude until you cannot see the horizon.
  7. Close Viewer Location.
4. Set Sun and Earth to correct size.
  1. On the Control Panel, open Planets.
  2. By default, the Earth and Sun are shown 5 times bigger than they should look, given how far away they are, how big they are, and how much zoom we are using. The Planetarium magnifies them so they look a reasonable size in the sky. The Planetarium never magnifies our home planet, and that is currently the Moon. There cannot be an eclipse if, from Earth, the Sun in the sky looks bigger than the Moon.
  3. Therefore we must set Sun/Earth Magnification to 1, meaning no magnification.
  4. Close Planets.
  5. Open Stars.
  6. Set Brightness Boost down to about 10. Brightness Boost makes dim stars brighter and bright stars bigger. We are reducing it just so we won't see any stars that look ridiculously large compared to the Earth.
  7. Close Stars.
Note: Sun/Moon/Earth magnification is explained more fully in the Control Panel tutorial.

5. Clean up.

  1. On the Control Panel, open Screen Decorations by left-clicking on it.
  2. Turn Lines, Labels, and Direction Markings OFF. We will leave On-Screen Controls ON.
  3. Close Screen Decorations by left clicking on the heading again.
  4. Close the Control Panel by right clicking..
Exercise: Watch the eclipse for yourself.
  1. Adjust zoom so the earth is a good size.
  2. Adjust your view so about the bottom third of the Earth is below the middle of the screen. (You can adjust this and the zoom more during the eclipse.)
  3. Under Date and Time on the Control Panel, press Start Clock.
  4. Close the Control Panel and watch the eclipse.
  5. When it is over, stop the clock again and set the time back to 17:00 hours. Watch the eclipse again without starting the clock, by stepping through it minute by minute.
Quiz – after you watch the eclipse:
  1. In which direction does the eclipse move across the Earth? Why?
  2. How does the eclipse start and end?
  3. From the Moon, what does the path of the eclipse look like?
  4. What does the path look like on a flat map of the Earth? Can you explain the difference?
You might also try watching the eclipse with the Earth as your home planet, from some distance above it. Then you can adjust your location to be more or less over the eclipse as it goes on.

6. Watching a Solar Eclipse from the Moon.

This is intended as a standalone video for YouTube, so at the beginning there will be some repetition of things you already know. And "link in the description" applies on YouTube, not here.

  1. This is the Dogulean Planetarium, a web page containing an interactive solar system simulation, written using the three.js WebGL library. You can find the Planetarium at dogulean.com, along with videos showing how to use it. There is a link in the description below.
  2. I have already set up almost everything to watch the Total Solar Eclipse of July 2, 2019, FROM THE MOON!
  3. We want to watch the eclipse from the Moon, because from there we can watch the whole course of the eclipse and we will not just learn, but actually see a lot about how an eclipse works.
  4. It is 16:30 hours, UTC or GMT, on July 2, 2019, with the clock stopped. We are on the Moon, looking at the Earth, which is right in front of us.
  5. To see that we are on the Moon, let's LOOK DOWN and then increase our altitude until we can see the whole thing. The Sun is behind the Moon and we are in the Moon's shadow, but the Moon is faintly lit by the glow of the full Earth, which is behind us.
  6. Let's go back down to the surface now and look back at the Earth by selecting it on the View Planet menu. Let's get a good view by zooming in. Let's also position the Earth so the path of the eclipse across the southern hemisphere, from about here to about here, is near the middle of the screen.
  7. Lets open the the Control Panel by right-clicking the mouse.
  8. We have set the clock rate to 60 so that, once we start the clock, one minute of real time will correspond to one hour of planetarium time. That way the eclipse, which ran for about 3 1/2 hours from beginning to end in real life, will only take us about 3 1/2 minutes to watch.
  9. Now let's start the clock and close the Control Panel by right clicking again.
  10. The Moon always casts a shadow in space. When that shadow touches the Earth, we call it a solar eclipse. The shadow moves across the Earth from West to East, because that is the direction in which the Moon moves around the Earth. Meanwhile, the Earth also turns from West to East, much faster than the Moon moves, but not as fast as the Moon's shadow moves.
  11. We see a black spot appearing at the edge of the earth. In that area, the core of the Moon's shadow is starting to touch the Earth and the Sun is coming up totally eclipsed. That is how a total solar eclipse usually starts, and it usually ends with the Sun going down somewhere, totally eclipsed, as the shadow slips off the earth.
  12. From our point of view, the area of totality will move across the Earth in a straight line. If you hold a sheet of paper over the video at the proper angle, the eclipse will move along one edge. But if you look at maps of eclipse paths, they are always curved. That happens because the Earth is curved and tilted. This part in the middle of our straight line is farther north than the two ends. If we drew the path on a flat map of the earth, our straight line would make a shallow, upside-down U.
  13. From our point of view, the eclipse moves along its path at a more-or-less constant rate. But from the Earth, at the two ends of its path, the eclipse travels faster along the ground, basically because there the ground is at more of a slant to our straight line. Therefore, at the ends, it takes less time from start to end of totality, as is appears from a spot on the ground, than it does in the middle.
  14. At only 3 1/2 hours, our eclipse will be rather brief. It takes place in the Southern Hemisphere in early July, at the beginning of winter, and the Eclipse is short for the same reason that the days are short at that season.
  15. If you look up local times for the start and end of the eclipse, the difference is much bigger than 3 1/2 hours. That is because the eclipse moves east, crossing many time zones, and local time increases by an hour each time it enters a new time zone.
  16. At this point, we get a good view of the shadow because it is mostly over clouds. The shadow is dark at the center where the eclipse is total, greyer outside, where it is only partial, fading farther away, where less of the Sun is eclipsed.
  17. Totality will continue until the dark part of the shadow falls off the east edge of the Earth's disk. And now, the Sun has set, totally eclipsed, somewhere over Argentina.
  18. And that's the end of the total solar eclipse of July 2, 2019.