A transit of Earth across the Sun as seen from Mars takes place when the planet Earth passes directly between the Sun and Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Earth could be seen from Mars as a small black disc moving across the face of the Sun.
Conjunctions
Transits of Earth from Mars usually occur in pairs, with one following the other after 79 years; rarely, there are three in the series. The transits also follow a 284-year cycle, occurring at intervals of 100.5, 79, 25.5, and 79 years; a transit falling on a particular date is usually followed by another transit 284 years later. Transits occurring when Mars is at its ascending node are in May, those at descending node happen in November. This cycle corresponds fairly closely to 151 Mars orbits, 284 Earth orbits, and 133 synodic periods, and is analogous to the cycle of transits of Venus from Earth, which follow a cycle of 243 years (121.5, 8, 105.5, 8). There are currently four such active series, containing from 8 to 25 transits. A new one is set to begin in 2394. The last series ending was in 1211.
View from Mars
No one has ever seen a transit of Earth from Mars, but the next one will take place on November 10 2084, and could be observed by future Mars colonists. The last such transit took place on May 11 1984.[cite web]
date = 1984-May-11 04:30 UT
title = Transit of the Sun by Earth from the center of Mars
publisher = [http://space.jpl.nasa.gov/ JPL Solar System Simulator]
url = http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=399&vbody=499&month=5&day=11&year=1984&hour=04&minute=30&fovmul=1&rfov=0.5&bfov=30&porbs=1
accessdate = 2008-06-06 [http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=399&vbody=499&month=5&day=11&year=1984&hour=13&minute=00&fovmul=1&rfov=0.5&bfov=30&porbs=1 (end of transit)] ] During the event, the Moon could almost always also be seen in transit, although due to the distance between Earth and Moon, sometimes one body completes the transit before the other begins (this last occurred in the 1800 transit, and will happen again in 2394).
View from Earth
A transit of Earth from Mars corresponds to Mars being perfectly uniformly illuminated at opposition from Earth, its phase being 180.0° without any defect of illumination. During the 1879 event, this permitted Charles Augustus Young to attempt a careful measurement of the oblateness (polar compression) of Mars. He obtained the value 1/219, or 0.0046. This is close to the modern value of 1/154 (many sources will cite somewhat different values, such as 1/193, because even a difference of only a couple of kilometers in the values of Mars' polar and equatorial radii gives a considerably different result).
In fiction
A 1971 science fiction short story by Arthur C. Clarke, "Transit of Earth", depicts a doomed astronaut on Mars observing the 1984 transit (setting the story in 1984 turned out to be too optimistic). The short story was first published in the January 1971 issue of "Playboy" magazine.
Dates of transits
Simultaneous transits
The simultaneous occurrence of a transit of Venus and a transit of Earth is extremely rare, and will next occur in the year .
ee also
* Astronomical transit
* Transit of Phobos from Mars
* Transit of Deimos from Mars
References
* Albert Marth, "Note on the Transit of the Earth and Moon across the Sun’s Disk as seen from Mars on November 12, 1879, and on some kindred Phenomena", Monthly Notices of the Royal Astronomical Society, 39 (1879), 513–514. [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1879MNRAS..39..513M]
* Andrew Crommelin, "Observations of Mars, 1904–6", Monthly Notices of the Royal Astronomical Society, 64 (1904), 520–521 [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1904MNRAS..64..506.&db_key=AST&page_ind=14&plate_select=NO&data_type=GIF&type=SCREEN_GIF]
* Jean Meeus & Edwin Goffin, "Transits of Earth as Seen from Mars", Journal of the British Astronomical Association, 93 (1983), 120–123 [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1983JBAA...93..120M]
* Charles Augustus Young, "Measures of the Polar and Equatoreal Diameters of Mars, made at Princeton, New Jersey, U.S.", The Observatory, 3 (1880), 471 [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?journal=Obs&year=1880&volume=3&page_ind=420&letter=.&type=SCREEN_GIF]
* cite book |author=Meeus, Jean |title=Transits|publisher=Willmann-Bell |location=Richmond, VA |year=1989 |pages= |isbn=0-943396-25-5
* [http://chemistry.unina.it/~alvitagl/solex SOLEX ]
External links
* [http://ssd.jpl.nasa.gov/horizons.html JPL HORIZONS System]
* [http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=399&vbody=499&month=11&day=7&year=2005&hour=10&minute=00&fovmul=1&rfov=1&bfov=30&porbs=1 Near miss of the Earth-moon system] (2005-Nov-07)
