OBSERVE MARS AT ITS BEST

October’s opposition of the Red Planet is the best opportunity to view the world for years to come. Pete Lawrence helps you make the most of it with a week-by-week guide to what may be visible on Mars’s disc

At long last Mars’s great opposition is upon us this month, and it’s time to make the most of its closeness to Earth, as well as its brightness and height in the sky. In this feature we’ll help you to maximise your telescope views of the Red Planet, with a guide to the Martian features that will be best to view each week. Two dates to mark on the calendar are when Mars comes closest to Earth, a distance of 62 million km, at 15:19 BST (14:19 UT) on 6 October, and a week later, on 13 October, when it reaches opposition. This year the Red Planet is also notable for reaching an altitude of around 42˚ from the UK and presenting a maximum disc size of 22.6 arcseconds; after 2020, we won’t see Mars’s apparent disc size exceeding 20 arcseconds again until 2033.

So, what will conditions be like on Mars during this opposition? It takes 687 days for the planet to orbit the Sun and go through its seasonal changes, experiencing two equinoxes and two solstices. A Martian equinox occurs when the Sun appears to cross Mars’s celestial equator, while solstices represent the instant when the Sun is at its most northerly or southerly position in the Martian sky. We divide Earth’s year into 12 months, but on Mars it’s usual to refer to periods in the Martian year (and its seasons) in terms of solar longitude (Ls). This value is measured in degrees from the planet’s northern hemisphere spring equinox.

The first Martian ‘month’ is considered to be the period between Ls=0–30˚, the second between Ls=30˚–60˚, and so on. The northern spring equinox at Ls=0˚ is followed by the northern summer solstice at Ls=90˚. The northern autumn equinox occurs at Ls=180˚ and the northern winter solstice at Ls=270˚. Between October 1 and 31, Ls increases from 288˚ to 310˚, Mars being in the grip of a northern winter, the southern hemisphere basking in summer sunshine. At this October’s opposition, Mars’s southern hemisphere will be tilted towards Earth.

When it comes to observing around opposition, telescopes will show us dark and light regions on Mars’s disc. These ‘albedo’ features are visible because of variations in reflectivity. The lighter regions generally represent desert areas, while the darker regions are exposed rock. In addition, the planet’s polar caps shine bright. The south polar cap (SPC) should dominate our view, but as it’s well into the southern summer, this will have shrunk to a fraction of its full size, the residual ice cap slightly offset from the planet’s axis of rotation. The north polar cap (NPC) is currently beyond the northern limb of the planet, but the shroud of cloud currently covering it should be partially visible. This north polar hood (NPH) will dissipate as Mars enters northern spring in late January and February 2021.

As we begin, let’s break the views up week by week during opposition month. Our descriptions are centred