Deimos transits the Sun, as seen by Mars Rover Opportunity on March 4 2004

A transit of Deimos across the Sun as seen from Mars takes place when Deimos passes directly between the Sun and a point on the surface of Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Deimos can be seen from Mars as a small black disc rapidly moving across the face of the Sun.

The event could also be referred to as a partial eclipse of the Sun by Deimos. However, since the angular diameter of Deimos is only about 1/10 of the angular diameter of the Sun as seen from Mars, it is more natural to refer to it as a transit. The angular diameter of Deimos is only 2 1/2 times the angular diameter of Venus from Earth during a transit of Venus from Earth.

A transit of Deimos from Mars usually lasts only a minute or two, due to its relatively rapid orbital period of about 30.3 hours.

Because they orbit Mars in low-inclination equatorial orbits, the shadows of Phobos or Deimos projected onto the surface of Mars exhibit a seasonal variation in latitude. At any given geographical location on the surface of Mars, there are two intervals in a Martian year when the shadows of Phobos or Deimos are passing through its latitude. During each such interval, zero or one transits of Deimos can be seen by observers at that geographical location (compared to about half a dozen transits of Phobos).

It is easy to see that the shadow always falls on the "winter hemisphere", except when it crosses the equator during the vernal equinox and the autumnal equinox. Thus transits of Deimos happen during Martian autumn and winter in the northern hemisphere and the southern hemisphere, roughly symmetrically around the winter solstice. Close to the equator they happen around the autumnal equinox and the vernal equinox; farther from the equator they happen closer to the winter solstice.

Because it orbits relatively close to Mars, Deimos cannot be seen north of 82.7°N or south of 82.7°S; such latitudes will obviously not see transits either.

On March 4 2004 a transit was photographed by Mars Rover Opportunity, while on March 13 2004 a transit was photographed by Mars Rover Spirit. In the captions below, the first row shows Earth time UTC and the second row shows Martian local solar time.

March 4 2004 transit from Opportunity

03:03:43 10:28:17	03:03:53 10:28:27	03:04:03 10:28:36	03:04:13 10:28:46

March 13 2004 transit from Spirit

00:04:27 13:54:11	00:04:37 13:54:20	00:04:47 13:54:30	00:04:57 13:54:40	00:05:07 13:54:50	00:05:17 13:54:59	00:05:27 13:55:09	00:05:37 13:55:19

The data is the tables below is generated using JPL Horizons. There is some discrepancy of a minute or two with the times reported for the series of images above. This may be due to imprecision in the ephemeris data used by JPL Horizons; also the JPL Horizons data gives local apparent solar time while the times reported above are probably some form of mean solar time (and therefore some of the discrepancy would be due to the Martian equivalent of the equation of time).

Note: the data below are valid for the original landing sites. To the extent that the rovers have moved around on the surface, the parameters of the transits as actually observed may be slightly different.

Near misses are indicated with strikeout.

See also

• Transit of Mercury from Mars
• Transit of Venus from Mars
• Transit of Earth from Mars

• Transit of Phobos from Mars

• Time and date and astronomy on Mars

Reference

• J. Bell, M. Lemmon, M. Wolff, Transits of Mars I and II, IAU Circ., 8298, 2 (2004). [1] (TeX DVI file is at [2]).

External links

• JPL Horizons (must use telnet interface for non-Earth observation points)

• Opportunity image gallery: Sol 39 (small images of the March 4 2004 transit are near the bottom of the page).
• Spirit image gallery: Sol 68 (small images of the March 13 2004 transit are near the bottom of the page).
• Spirit image gallery: Sol 420 (small images of the March 9 2005 transit are near the middle of the page).
• Animation of March 4 2004 transit