The fourth planet is a little more than half as wide as Earth. It is one and a half times farther out from the Sun than we are, and takes nearly twice as long as we do to travel around its orbit.
It has roughly alternating “good” and “bad” years for observation. They should more objectively be called opposition and non-opposition years. In the favorable years, we are nearer to Mars, and overtake it at opposition: the stage where it appears directly outward from the Sun and therefore high in our skies at midnight, nearest, largest, and brightest.
The oppositions themselves vary in favorability, in a cycle of about 9 around the orbit..
These are the numbers that describe Mars:
| mean distance from Sun | 1.522 AU | |
| sidereal period | 1.89 years = | 687 days |
| synodic period | 2.13 years = | 780 days |
| eccentricity of orbit | 0.093 | |
| inclination of orbit to ecliptic | 1.9° | |
| diameter | 6,790 km | |
| satellites | 2 |
An astronomical unit (AU) is the average Sun-Earth distance, 149,597,871 kilometers or 92,955,807 miles.
The relation between distance and period follows from Kepler’s third law of planetary motion: period squared equals average distance cubed. (In which the period is in Earth years and the distance is in astronomical units or Sun-Earth distances.
Mars’s orbit
We are looking from a viewpoint 35° north of the ecliptic plane, and 6 AU from the Sun. The paths of Earth and Mars are shown for one year. The planet globes are at the beginnings of months, and are exaggerated 300 times in size, the Sun 5 times. Stalks connect Mars northward or southward to the plane of the ecliptic.
The boundaries of the 12 constellations of the zodiac, and the planes of the ecliptic and the celestial equator, are drawn on an imaginary sphere, centered on the Sun, with radius 2 AU.
The dashed curve is a circle at the mean distance of Mars from the Sun.
The vernal equinox direction, where the celestial equator cuts northward through the ecliptic, is the zero or origin point for mapping the sky.
Perihelion and aphelion are the minimum and maximum distances of the planet from the Sun. The ascending and descending nodes are the points at which the planet’s orbit passes northward and southward through the plane of the ecliptic. The northernmost and southernmost latitudes are where the planet is farthest from the ecliptic plane.
I draw the line to the perihelion point thicker as a reminder that perihelion is a favorable stage for observation. And the northernmost latitude is also favorable (for north-hemisphere observers).n.
Roughly alternating years
On our inside track, we take 2.13 years to catch up with Mars and pass it at the next opposition. So the oppositions are in alternate years, moving later through the year by an average of about 50 days, until they skip a year. For instance, an opposition comes near the end of 2022, so the next is not till early 2025.
2020 Oct 13
2022 Dec 8
2025 Jan 16
2027 Feb 19
2029 Mar 25
2031 May 4
2033 Jun 28
2035 Sep 15
2037 Nov 19
2040 Jan 2
The cycle of oppositions
The oppositions are spaced around the sky roughly 1/7 of the circle apart, in a cycle of slightly more than 15 years.
