Mars and Earth are getting up close and personal, but there’s no need to worry about having bits of Mars fall into your garden or having to wipe extra Mars dust off your primary mirror, unless you live on Mars, for the rest of us it’s still a long way away. Earth (365.25 days) roughly whizzes around the sun about twice as quick as Mars (687 days) so every couple of years we catch up with the red planet and our orbits line up with the sun, called the synodic orbit, 780 days. This year, this will happen around 31 July/1 Aug depending where you are on the planet. What does this mean? It means that Mars will be brighter than normal and great for viewing in telescopes and there’ll be a good chance of being able to spot a little more detail than when we’re not on the same side of the Sun and as close.
Planet orbits are not a simple thing, they are not perfectly circular so the planets are not always the same distance from the Sun. The shape of the orbits are a bit of an ellipse and this ellipse slowly precesses around the sun over many hundreds of years. The orbit of Mars has quite a bit of eccentricity, much more that the Earth, so the distance between the Earth and Mars at the closest point each orbit can be significantly different.
For each orbit of the Sun, planets pass through a point in which they are closest to the Sun called the perihelion and a point in which they are furthest from the Sun, called the aphelion. This year Mars will reach the point at which it is closest to the Sun on 17 Sep which is not too many weeks after the point at which Earth catches up to Mars in its orbit. Back in 2003 Mars and Earth lined up with the Sun almost when Mars was at its closest to the Sun, this put the Earth and Mars as close to each other as they had been in 60,000 years at about 56 million kilometres.
It won’t be as exciting this time around but still only about 58 million kilometres apart. When Mars is the farthest from the Sun (the aphelion) and the Earth gets between it and the Sun, the minimum distance is about 100 million kilometres so getting between Mars and the Sun at Mars’ perihelion makes quite a difference.
At opposition Mars will appear about 24.3 arcseconds in size as compared to the Moon which will be a whooping big 29 arcminutes. For a few weeks either side of the of the closest point, Mars will be the fourth brightest object in the night sky, just knocking Jupiter off that position for a short time. There will be the Facebook and Twitter hoaxes that will claim Mars will appear as big as the Moon so watch out for those, and have a laugh when your friends tell you it’s true.