See Mercury make its best appearance in the evening sky of 2021

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Mercury, the closest planet to the Sun, is starting to make its best appearance in the evening sky of 2021 for observers at northern latitudes. Here’s how to see Mercury.

The crescent Moon and Mercury caught on the evening of 13 May, 2021, from Walmer, UK. Image credit: Paul Sutherland

Because it lies near the Sun, Mercury never strays far from it in the sky. That means it can only usually be seen soon after sunset or shortly before sunrise, depending on which side of the Sun it lies.

In May 2021, Mercury will lie to the east of the Sun, and so will follow it across the sky, finally setting shortly after the Sun does.

The ecliptic – the plane of the planets’ orbits around the Sun – will be at a steep angle to the horizon from mid-northern latitudes. This will lift Mercury higher above the horizon than usual, making it easier to spot.

As May opens, Mercury will be at a bright magnitude of -1. It may be visible as it gets dark, around 45 minutes after sunset, at around five degrees above the WNW horizon.

Planet Mercury photographed alongside the windmill at Ripple, Kent, UK, on the evening of February 24, 2019. Image credit: Paul Sutherland

Over subsequent nights, Mercury’s brightness will slightly fade, but it will move higher in the sky, so making it easier to spot. If you do have clear skies and an unobstructed horizon, such as the ocean, you may see Mercury glide past the Pleiades star cluster on the 4th, but binoculars or a telescope will be essential to pick out the cluster.

In the second week of May, Mercury will fade from -0.5 to zero magnitude, but 45 minutes after sunset it will still be at an elevation of around 10°.

Between Mercury and the Sun will lie the planet Venus, shining at a brighter magnitude of -3.8, but its lower elevation may make it harder to spot if your horizon is obstructed.

Make sure you look out on the evening of 13 May, when the young crescent Moon will lie close to Mercury. If you have a good clear view, this could make a great photo opportunity.

Mercury will lie farthest from the Sun, an event called Greatest Elongation East, on May 17. (They will lie 22° apart, meaning you could fit 44 full Moons between it and the Sun in the sky then!)

How Mercury, Venus and the slim crescent Moon will be arranged in the WNW sky about 45 minutes after sunset on 13 May from mid-northern latitudes. Image produced using Stellarium.

Mercury will have faded to magnitude 0.5 by this date, but will not set until around two hours after the Sun, so you will have a good chance to see it in a darkening twilight sky, an hour or more after the Sun has set.

It will be as bright as some of the brightest stars in the sky, though an effect called extinction will dim it slightly, due to the greater depth of Earth’s atmosphere between it and the observer near the horizon.

You will be seeing it when it is only a few degrees above the horizon, so any low clouds in the distance will hide it. You will also need to make sure you have a clear horizon, unobstructed by trees or buildings.

Binoculars will make Mercury easier to spot, but make sure you only use them after the Sun has set. The sky will not be dark enough anyway, and you risk damaging your eyes if you accidentally point them at the Sun.

Through a large and powerful telescope, Mercury will resemble a half moon in mid-May, because the inner planets, Mercury and Venus, show phases just like the Moon does.

The planet reaches inferior conjunction, when it will lie between the Earth and the Sun (though not directly in line) on 11 June, 2021. Its next appearance in the evening sky will be in September, 2021, which will be far more favourable for southern hemisphere observers.

Mercury is a rocky planet but has a diameter of only 4,880 km, which is less than Ganymede and Titan, the largest moons of Jupiter and Saturn. Read more about Mercury.

Related: How to observe Mercury

Related: What’s in the night sky this month


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