Chart showing Mars oppositions from 2018 to 2033

Mars oppositions are not created equal

The diagram above – from the Royal Astronomical Society of Canada (RASC) – shows why Mars varies in brightness from one opposition to the next. The diagram represents the orbits of Earth and Mars as viewed from above the solar system. RASC explains:

Straight lines link simultaneous positions of the two planets for eight successive oppositions of Mars, beginning with that of the year 2018. The separation of the two planets in astronomical units (Earth-sun units, or AU) at the various oppositions is indicated beside each of the connecting lines.

The months inside of Earth’s orbit indicate the position of Earth during the year (both planets orbit counterclockwise).

For each orbit, two tick marks labeled A and P indicate the aphelion point and the perihelion point, respectively. The direction of the vernal equinox is shown (toward the late-September position of Earth). Around the orbit of Mars is indicated its declination (ranges between +27° and 28°) and the constellation in which Mars resides when at opposition.

More info about this chart

The RASC continued:

Four views of Mars are shown: at its two equinoxes and two solstices. These views show the portion of Mars illuminated by the sun, the location and approximate size of its north polar cap, and the apparent size of Mars (labeled in arcseconds) for oppositions occurring at these points in its orbit.

The seasons of the Martian northern hemisphere are indicated around the outer margin of the diagram, and are very nearly one season ahead of those on Earth at the same orbital position. (For the southern hemisphere of Mars, the season and the configuration of the south polar cap are the same as those of the diametrically opposite view.) Note that the maximum angular diameter Mars can attain (25 arcseconds) occurs near its perihelion, at a late-August opposition.

As an example of the information that can be read from this diagram: The next opposition of Mars occurs in mid-January 2025 with Mars located near declination +25° near the Cancer-Gemini border, 0.64 AU from Earth, and about 15 arcseconds in diameter. It will be spring in the Martian northern hemisphere and the north polar cap will be obvious.

The 2025, 2027, and 2029 oppositions of Mars occur with Mars near aphelion and Earth not far from perihelion, thus they are not favorable. They do occur with Mars north of the celestial equator, good for observers in mid-northern latitudes, but at the times of those three oppositions the observing weather can be cold and cloudy.

Bottom line: Here’s a classic Royal Astronomical Society of Canada (RASC) diagram by Roy L. Bishop. It shows Mars oppositions from 2018 to 2033. The diagram lets you see why Mars is brighter at some oppositions than others … And we are honored to host it here.