Transit of Earth from Saturn
Encyclopedia
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. If Saturn reaches opposition too far from a node there will be no transit. Most passages of Saturn through one of its nodes, which take place every 14–15 years, result in a transit of the Earth.
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 387.107 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.
The images correspond to a hypothetical observer at the center of 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.
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
of Earth across the Sun as seen from Saturn takes place when the planet Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
passes directly between the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.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
Opposition (astronomy)
In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place . In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.The astronomical symbol for opposition is ☍...
while it is near one of the nodes
Orbital node
An orbital node is one of the two points where an orbit crosses a plane of reference to which it is inclined. An orbit which is contained in the plane of reference has no nodes.-Planes of reference:...
. The transits generally happen on the day of opposition, but a few months before or after passage through a node. If Saturn reaches opposition too far from a node there will be no transit. Most passages of Saturn through one of its nodes, which take place every 14–15 years, result in a transit of the Earth.
The event is particularly interesting because both the Earth and the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
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
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....
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 387.107 days. It can be calculated using the formula 1/(1/P-1/Q), where P is the sidereal orbital period
Sidereal year
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It was equal to at noon 1 January...
of Earth (365.25636 days, not the same as a tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...
) and Q is the orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, 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.
The images correspond to a hypothetical observer at the center of Saturn. Since Saturn has a large radius, the parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
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 | http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=3&vbody=6&month=1&day=14¢ury=20&decade=0&year=5&hour=1&minute=0&fovmul=1&rfov=0.25&bfov=0.25&porbs=1 |
| July 20, 2020 | |
| July 16, 2049 | |
| January 16, 2064 | |
| July 11, 2078 | |
| January 9, 2093 |