Don’t miss Mercury in the eastern sky at dawn during August 2019

By Ade Ashford

Between about 8 and 20 August 2019, early risers can try their luck at spotting Mercury low in the east-northeast at the start of civil twilight, currently around 42 minutes before sunrise for the heart of the UK. The time of civil dawn (when the Sun is 6° below the horizon) advances a couple of minutes each successive day at UK latitudes, but this looping animation shows how the innermost planet’s position relative to the horizon in the British Isles remains almost constant, peaking in altitude around 14 August. The degree scales left and right show height above the horizon; 10° is about the span of a fist at arm’s length. AN animation by Ade Ashford.
Mercury attains a greatest elongation 19 degrees west of the Sun on 9 August 2019, hence the innermost planet is a morning object. This means that early risers in the British Isles with clear skies have several opportunities to view Mercury between the first and third weeks of this month.

The innermost planet has a reputation for being elusive, but the secret to catching a glimpse of it is mainly timing. For this particular apparition, observers in the UK need to be scanning the east-northeast around the onset of civil twilight when the Sun is 6 degrees below the horizon.

For the heart of the British Isles, civil dawn is currently about 42 minutes before sunrise. You can find out when civil dawn occurs for where you live by selecting your nearest city in our interactive online Almanac. Click here for an Almanac user’s guide.

If you try observing earlier than civil dawn then Mercury will be too low to see but the sky will be darker, while leaving it too late means that the little planet will be higher but the sky is too bright. (Never be tempted to sweep with binoculars or a small telescope with an imminent sunrise, risking irreparable damage to your eyesight.)

Mercury passes from the constellation of Gemini into Cancer on the morning of Friday, 9 August. The planet crosses a line drawn from Castor through Pollux (the two most prominent stars in Gemini), about a span of a fist at arm’s length below Pollux on the mornings of the 11th and 12th. Note that Mercury is highest in the UK sky around civil dawn on 14 August. The planet is also brighter towards the end of the period, exceeding magnitude -1.0 on the 19th.