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Earth-Moon Geometry

graphic of geometry between Earth, Moon and EPOXI on 29 May Click for full size image.

Caption: Diagram showing the geometry of the Earth, Moon and EPOXI at the time of the Lunar transit movie.

The top panel shows the Earth's trajectory or path in blue dashed lines and EPOXI's spacecraft in white dashed lines, as seen from above the solar system, in their relative positions at the midtime of the Lunar transit movie. The spacecraft was 31 million miles (50 million kilometers, 1/3 AU) from Earth. The orbit of the moon is about the size of the blue circle marking the Earth's position. The Sun is out of the field of view to the upper right, as shown by the white arrow pointing to the circled dot (sun symbol) in the lower panel.

The lower panel shows the Earth-Moon system expanded (so EPOXI is out of the frame) to show the Moon's orbit (yellow). The positions of the Moon at the start and end of the sequence are marked, showing the motion of the moon over the 24-hour period captured in the movie. At the midpoint time (08:00 on May 29), the Moon is between the Earth and the EPOXI spacecraft.

The start and end marks are for the start and end of the movie sequence. So the start mark shows the moon's position at 20:00 May 28, and the end mark is for 20:00 on May 29. The actual transit, where the moon is visible in the frame, only covers the part of the sequence from about 6:00 to 10:00 on May 29, with just the Earth shown before and after.

For reference, there is also the diagram of the mission trajectory.

Credit: NASA/JPL-Caltech/UMD/GSFC/Tony Farnham

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