Transit of Earth from Saturn
A transit of Earth across the Sun as seen from Saturn takes place when the planet Earth passes directly between the Sun and Saturn, obscuring a small part of the Sun's disc for an observer on Saturn. During a transit, Earth can be seen from Saturn as a small black disc moving across the face of the sun.
A transit of Earth happens when Saturn is in opposition while it is near one of the nodes. The transits generally happen on the day of opposition, but a few months before or after passage through a node. Most passages of Saturn through one of its nodes, which take place every 14–15 years, result in a transit of the Earth. In fact if Saturn reaches opposition very close to the passage at a node, as in 2005 and 2049, the transit will be almost central. However, if Saturn reaches opposition too far from a node there will be no transit.
The event is particularly interesting because both the Earth and the Moon can usually be seen together in transit. In rare cases one of them transits and the other does not, because there is a grazing transit for one and a near-miss for the other. In other cases, the two may be so close together that they are hard to tell apart, such as on July 20, 2020, when the Earth and Moon are less than 5" apart at mid-transit.
Naturally, no one has ever seen a transit of Earth from Saturn, nor is this likely to happen in any foreseeable future. The last one took place on January 13–14, 2005—though the Cassini probe was present in the Saturn system, it was also the day of the Huygens probe mission. Furthermore, the angular resolution needed to capture the occultation was near the limits of Cassini's imaging subsystem, to say nothing of the concerns of pointing the probe's camera directly at the Sun.
A transit could be observed from the surface of one of Saturn's moons rather than from Saturn itself. The times and circumstances of the transits would naturally be slightly different.
The Earth-Saturn synodic period is 378.107 days. It can be calculated using the formula 1/(1/P-1/Q), where P is the sidereal orbital period of Earth (365.25636 days, not the same as a tropical year) and Q is the orbital period of Saturn (10746.940 days).
Note: the images linked to in the following table do NOT take into account the finite speed of light. The distance of Earth from Saturn at inferior conjunction is approximately 8.5 AU, which would correspond to about 70 light-minutes. It can take up to 12 hours for Earth to transit across the Sun at its widest point, thus the images correspond fairly closely to what would actually be seen by an observer on Saturn.
Since Saturn has a large radius, the parallax of Earth between Saturn's center and its north or south pole would be about 9.7", which is about 4.5 times Earth's apparent angular diameter of 2.2", or about 4.5% of the Sun's angular diameter (about 3.5'). Therefore, some extremely close near-misses might be seen as grazing transits at Saturn's poles.
|Transits of Earth from Saturn|
|July 13, 1931|
|January 12, 1946|
|July 19, 1961|
|July 14, 1990|
|January 13, 2005|||
|July 20, 2020|
|July 16, 2049|
|January 16, 2064|
|July 11, 2078|
|January 9, 2093|
- C.T. Whitmell, Saturn visible through the Cassini Division, Monthly Notices of the Royal Astronomical Society, 62 (1902), 457