Ken Tapping, 28th October, 2015
These mornings, in the early hours when the eastern horizon is just beginning to brighten, you will, if it is clear, see a moderately bright, starlike object close to the horizon. It is the planet Mercury, the closest planet to the Sun and the most elusive one to see. Higher in the sky lies Venus, second planet out from the Sun and incredibly bright, Nearby is Jupiter, fifth planet from the Sun, a bit fainter and yellowish. Close by, but much fainter is Mars, fourth planet from the Sun. Mars is faint because it is currently on the other side of the Sun.
Mercury lies only about 40% of the Earth’s distance from the Sun, so from where we are it is never far from the Sun in the sky. This also means its orbital period – how long Mercury takes to complete a trip around the Sun – is only 88 days, compared with the Earth’s roughly 365 days. Its rapid motion in the sky led to the planet being named after the messenger of the Roman Gods. This rapid motion also means that it does not stay at a good separation angle from the Sun for very long. Within a matter of weeks it drops back into line with the Sun. If we want to see Mercury, we need to see it this month, or wait until December, when it will be on the opposite side of its orbit, and visible low in the western sky after sunset.
The best way to easily find Mercury and then to safely observe it is to use the horizon to block out the Sun. If Mercury is to the west (right) of the Sun, then we look for it in the east before sunrise, and if it is to the east (left) of the Sun, look for it low in the west after sunset. However, Mercury never rises more than about 90 minutes before the Sun or sets more than 90 minutes after, so we never see it in a fully dark sky and never far above the horizon. Many don’t ever get an opportunity to see this elusive planet, so here’s a chance to try.
Mercury is bright enough to observe during the day, but not with your unaided eyes. However, you can see it with a telescope or binoculars if you know exactly where to look. Since this world is never far in the sky from the Sun, it is extremely dangerous to “sweep” for it with binoculars or a telescope. It is far too easy to accidentally scan across the Sun. Even a small fraction of a second of looking at the Sun through binoculars or a telescope will lead to permanent eye damage. Only sweep for Mercury when the Sun is completely below the horizon.
At the Earth’s distance from the Sun, about 1400 Watts of solar energy hits every square metre of the top of our atmosphere. Mercury gets hit with about 8800 Watts per square metre. Not surprisingly, it is hot. In the middle of the daytime side of the planet, where the Sun is overhead, the temperature reaches around 430 C. During the night the temperature at that location falls to about -170C. What might be surprising though is that there are craters near the poles where the Sun has never shone, and there we find what may well be among the coldest places in the Solar System.
Our world has a dense atmosphere. This acts like a woolly blanket around the planet, evening out the temperatures, and of course the wind and weather tend to further smooth out the temperature differences. Mercury has almost no atmosphere. Heat will only reach places on that world’s surface if the Sun is shining directly on them, or being reflected from something else. Places that are never reached by direct or reflected sunlight and free to radiate heat off into space will get very cold. One might expect billions of years of freezing and roasting would dry Mercury out. However, when the Messenger spacecraft orbited around the planet between 2011 and 2015, it found ice in some of the coldest and darkest locations. It’s unlikely this is Mercury’s last surprise for us.
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