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The apsides refer to the farthest (2) and nearest (3) points reached by an orbiting planetary body (2 and 3) with respect to a primary, or host, body (1). An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides / ˈ æ p s ɪ ˌ d iː z / AP-sih-deez) [1] [2] is the farthest or nearest point in the orbit of a planetary body about its primary body.
Mars comes closer to Earth more than any other planet save Venus at its nearest—56 million km is the closest distance between Mars and Earth, whereas the closest Venus comes to Earth is 40 million km. Mars comes closest to Earth every other year, around the time of its opposition, when Earth is sweeping between the Sun and Mars. Extra-close ...
The ancient Greek astronomer Hipparchus noted the apsidal precession of the Moon's orbit (as the revolution of the Moon's apogee with a period of approximately 8.85 years); [4] it is corrected for in the Antikythera Mechanism (circa 80 BCE) (with the supposed value of 8.88 years per full cycle, correct to within 0.34% of current measurements). [5]
=, the direction to the planet as seen from the Sun, the true anomaly. The problem is to compute the polar coordinates (r,θ) of the planet from the time since perihelion, t. It is solved in steps. Kepler considered the circle with the major axis as a diameter, and
On both Earth and Mars, these two precessions are in opposite directions, and therefore add, to make the precession cycle between the tropical and anomalistic years 21,000 years on Earth and 29,700 Martian years (55,900 Earth years) on Mars. As on Earth, the period of rotation of Mars (the length of its day) is slowing down.
Dwarf planet 90377 Sedna will reach its perihelion of 76 AU from the Sun. [34] 2079 August 11 Mercury occults Mars, the first since at least 1708. [31] 2083 A star system known as "V Sagittae" is expected to go nova this year (+/- 11 years). 2084 November 10 Transit of Earth as seen from Mars, the first and the only one in this century. 2085 ...
At their furthest Mars and Earth can be as far as 401 million km (249 million mi) apart. [191] Mars comes into opposition from Earth every 2.1 years. The planets come into opposition near Mars's perihelion in 2003, 2018 and 2035, with the 2020 and 2033 events being particularly close to perihelic opposition. [192] [193] [194]
For Earth-orbiting satellites, the reference plane is usually the Earth's equatorial plane, and for satellites in solar orbits it is the ecliptic plane. The intersection is called the line of nodes, as it connects the reference body (the primary) with the ascending and descending nodes.