Mercury is the closest planet orbiting the sun in our solar system. It’s also the smallest of the eight planets, being about two-fifths the size of earth – smaller than some moons.
Being closer to the sun than the earth, Mercury is known as an inferior planet. Like all the planets of the inner solar system it’s made up of rock & metal, with a proportionally large molten core.
The surface of mercury is much like the moon, covered with impact craters. This is because there is no atmosphere and no geological activity to change the surface. It is a place of extreme temperatures, with a surface temperature around 427°C (800°F) during daylight to -173°C (-280°F) at night near the equator. The poles are always freezing, due to the fact that Mercury’s axial tilt is almost non existent, the least of any planet in the solar system.
A day on Mercury lasts a long time – 59 earth days. Yet the year is very short – Mercury takes just 88 earth days to orbit the sun. This extremely slow spin is unique in our solar system, with the other planets spinning at a much faster rate. These figures mean that Mercury has just 3 of its days every 2 of its years!
Another feature of Mercury is its accentuated orbit. While all the planets orbits are technically ellipses, they’re very close to circular. Only Mercury has a highly elliptical orbit. Mercury’s distance from the sun varies from 46,000,000 km to 70,000,000 km (28,580,000 mi to 43,500,000 mi). As Mercury’s orbit brings it closer to the sun it also speeds up, travelling faster in its orbit on the close side of the ellipse and slower on the far side. This, coupled with the very slow axial rotation, gives the sun a retrograde motion for part of the Mercurian day – the sun will actually go backwards in the sky.
Mercury is not as easily observed as the other planets due to its proximity to the sun. It will always be near the sun, appearing low in the sky just before sunrise or just after sunset, depending which side of the sun it’s on relative to us. For much of the year it’s not visible at all, and when it is it’s often not for long and not fully dark. Optimal viewing rarely occurs due to the number of factors that need to be right. This makes it a difficult target for beginning observers. Through a telescope Mercury will appear as a bright disc showing its phase with little other details observable.
Mercury goes through phases just as the moon and Venus do. It’s best viewed when it’s at its furthest distance from the sun as viewed from earth, for at this time it will be higher in the sky and more likely visible in full dark. This is known as greatest elongation. This will mean it will be in the first or last quarter phase, as from our perspective it will be directly out to either side of the sun. Mercury’s angular height above the horizon varies depending on which part of its orbit it’s in when it’s at greatest elongation. The highest will be when it’s at aphelion. This will occur at the March equinox for greatest western elongation or the September equinox for greatest eastern elongation.
Mercury’s orbit is at a 7° angle to the plane of earths orbit (the ecliptic) and this also has an effect on best viewing times and locations. Southern Hemisphere viewers have a better view of Mercury due to the angle Mercury will be at when at ephelion and the time of year it occurs – early autumn for greatest eastern elongation or late winter for greatest western elongation. For most northern hemisphere viewers Mercury will never be visible when it’s fully dark.