Spot Mercury and Venus

Moon, Venus. Mercury
The Moon, Venus and Mercury (far left, centre) on 19 March. Photo: Robin Scagell

Planet-spotters have had a lean time for several months, with no bright planets to be seen in the evening sky. But right now there are two to be seen, one of which is the elusive Mercury, making its best appearance in the evening sky for the year.

Being the closest planet to the Sun, Mercury never strays out of the twilight sky, and you have to be in the right place at the right time to catch it. So make the most of its appearance this month, as it won’t be as well placed in the evening sky for another year.

The direction to look is due west, and the time to start looking is about 30 minutes after sunset, which at this time of year is around 6 pm virtually everywhere in the country. Look in the brightest part of the twilight and as the sky gets darker you should be able to see a bright starlike point just a few degrees above the horizon.

You’ll need an unobstructed and clear horizon, and if there are any clouds around you might need to wait till they drift away. Binoculars will help, but once you’ve spotted Venus it’s easier to find it again. It will become easier to see as the sky gets darker, but lower down as well. Having found Venus, look above it and to its upper right and you should spot the fainter Mercury. Under no circumstances try looking before sunset – not only is the sky too bright then, but you could damage your eyesight by looking at the Sun.

The pair will be around for a couple of weeks, though they will change position slightly night by night. Mercury is at its farthest from the Sun, so easiest to spot, on 15 March, but a few nights later there will be the added spectacle of the thin crescent Moon in there as well. You might find it on the 18th, lower down than Venus and to its left, but on the 19th it will be higher than Venus and will make a great photo-opportunity. The diagrams below show the relative positions of Venus, Mercury and the Moon on selected occasions.

Using a telescope

To the naked eye or even binoculars both planets will appear just as dots. But if you have a reasonable telescope, capable of magnifying 50 times or more, take a look at them. Venus is currently on the far side of the Sun, and is almost fully illuminated so it appears as a disc. But Mercury is closer to us and moving towards us all the time. On the 13th you’ll see that it has a definite phase, like a half Moon, though about a third smaller than Venus (7 seconds of arc across, compared with 10 seconds for Venus). Bear in mind, though, that with the planets so low the turbulence of our atmosphere will make their images very unsteady, so unless you are lucky enough to get very calm conditions you won’t get a sharp view.

However, within a few days it’ll become more of a crescent as it moves towards us and more in line with the Sun. If you can keep track of it over the evenings you’ll see it become thinner and thinner, until on the 19th, when the crescent Moon will be there to help you find it, Mercury will also be a crescent, about 25% illuminated. Over the next few nights it gets closer to the Sun, thinner and fainter, until by about the 25th it will be very difficult to spot. The insets in the diagrams below show how Mercury’s diameter changes night by night. That of Venus remains nearly constant.

Mercury and Venus
Changing positions of Venus, Mercury and the Moon during March 2018 as seen from the UK. The top left inset shows the diameter and phase of Venus, which remains almost constant during this period, while the top right inset for each night shows the phase and diameter of Mercury for comparison.