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HeyWhatsThat Eclipse Notes

Be sure to visit the main site, whose mission is to tell you the names of the mountains you can see when you're standing almost anywhere in the world, and the general FAQ, which lists all the features HeyWhatsThat has to offer. Follow us on Twitter and sign up for our occasional email announcements.

If you're interested in the night sky, you may want to take a look at our planisphere (FAQ) site, where we overlay your horizon and the planets in the desktop version of Google Earth, or our cosmic site using Google Sky in the browser.

Layout

We display an image of the Earth on the left of the page, an image of the night sky on the right, and a timeline along the bottom.

The Earth and sky panes are Google Earth images provided by the Google Earth plug-in. You can drag and zoom the images. To the sky pane we add an horizon line and images of the Sun, Moon, Earth's shadow. (We also show an image of Venus for its 2012 transit, and an asteroid image for the February 2013 close approach of 2012DA14.) On the Earth pane we mark the current viewer location with a purple X. For total and annular solar eclipses, we also show the path of the Moon's shadow's umbra across the Earth's surface: the dark disk marks the area that sees the full eclipse at any given time.

On the timeline a vertical line marks the moment represented in the sky. For solar eclipses, we mark the timeline with a purple X if we can determine the maximum eclipse for the current viewer location.

Controls

Planet positions and timings

Position data for the Sun, Moon, planets and asteroids is obtained from the telnet service of NASA's Jet Propulsion Laboratory HORIZONS System. We use geocentric positions for the Sun and Venus, but topocentric positions -- based on the viewer location set in the Earth pane -- for the Moon and Earth's shadow and closely approaching asteroids.

For precise eclipse and transit timings, visit the NASA Eclipse web site. (Here are their pages for the May 20 solar eclipse and the June 6 Transit of Venus.)

Sun and Moon image details

The images of the Sun and Moon are fixed and do not reflect their current conditions or orientation. The Sun's surface is extremely dynamic; you can see current images at spaceweather.com. The precise area of the Moon visible during a full Moon varies as well.

The images of the Sun, Moon and Venus were found by browsing the NASA Planetary Photojournal, Wikipedia, and the Wikimedia Commons. We converted to PNG, cropped the planets and made the backgrounds transparent, and reduced sizes. The Moon image used for solar eclipses and the Venus image used for the transit are darkened from the original. The image used for the February 2013 close approach of asteroid 2012 DA14 is actually of another asteroid, Ida, and is shown about 10,000 times larger than 2012 DA14. The originals are here: Sun Moon (Credit: Luc Viatour www.lucnix.be) Venus Ida

For the Earth's shadow, we compute the radius of the Earth's shadow's umbra and penumbra at the distance to the Moon, and draw the penumbra with a 50% gray, 50% transparent PNG and the umbra with a black, 50% transparent PNG. In the computation we use the radius of the Earth at latitude 45° and apply Danjon's empirical rule of increasing the radius by 1/85 to account for the Earth's atmosphere. Our results come pretty close to the NASA Eclipse Web Site Lunar eclipse predictions.

Inside baseball: the Penumbra

Google Earth has trouble drawing filled polygons around either pole. So when a solar eclipse has a penumbra that will cross one of the poles, we use a thick line rather than a filled polygon to represent the penumbra.

It also has trouble filling areas that cross the international date line (e.g. longitude changes from -180 to 180); you can see how we handle that in the source in the function denormalize().

2/2013