Mars passes close to aphelic opposition

Mars Orbital Path
Diagram showing Mars and Earth orbits
Credit: Jeffrey D. Beish

The planet Mars is currently closer to the Earth than it has been since early 2010, and it will not be this close again for over 2 years. Because the Earth has a shorter orbital period than Mars, at around 365 days as opposed to 687, we find ourselves passing between Mars and the Sun on a regular basis. On average this happens every 2 years and 50 days, and when it does we see Mars in the opposite direction to the Sun - an event called an opposition. During the time of opposition, Mars appears much larger in the night sky, which means that observers have a better chance of obtaining a good image of it using their own telescopes; and by using the Liverpool Telescope, we have a good chance of being able to see surface features on the red planet, such as the polar regions, or the darker central regions that were once thought to be Martian Seas.

Because Mars and Earth have elliptical orbits, the two extremes of Martian oppositions are categorised as being either perihelic or aphelic; that is to say, they can happen when the alignment of Earth and Mars coincides with Mars' nearest or farthest distance from the Sun - points known as perihelion and aphelion. The current opposition is very close to the type known as aphelic, which means that although Mars is close to Earth, it is still around twice the distance (and half the size) that it was during the perihelic opposition back in 2003. However, because perihelic oppositions only occur during our winter, the tilt of the Earth means that Mars appears much lower over the horizon than it does at the moment, which in turn means that we get a much clearer view of an otherwise smaller Mars.

In order to help you picture what is going on, why don't you use the NSO's Electric Orrery to see how the current aphelic opposition differs from the perihelic opposition that occurred back in January 2003.

If you manage to get any good observations of Mars in the coming weeks, we would love to hear from you, so please send us an image with some details that we can feature on the website, and on the NSO's new facebook page.