Skywatchers across the United Kingdom are in for a celestial treat this December, as the elusive planet Mercury makes one of its best appearances of the year. The innermost planet, often lost in the Sun's glare, will be visible in the pre-dawn twilight, offering a rare viewing opportunity for early risers.
A Favorable Celestial Alignment
The viewing chance arises from a specific planetary configuration known as greatest western elongation. This occurs when Mercury, from our perspective on Earth, appears at its maximum angular distance west of the Sun. For observers, this means the planet is positioned far enough from the Sun's bright light to be seen in the morning sky before sunrise.
The geometry is particularly favourable for those at mid-northern latitudes, including the UK. The optimal viewing window is a narrow one, between 30 to 60 minutes before sunrise. During this brief period, Mercury will be low in the south-eastern sky but shining unmistakably bright against the twilight.
How and When to Spot Mercury
For observers in London, the view looking south-east at 7am GMT on 7 December will reveal Mercury. As night transitions to day, stars will gradually fade, but the planet will remain visible, the last pinprick of light to succumb to the dawn.
Successful viewing requires a clear, unobstructed view of the eastern horizon. While binoculars can aid in initially locating the planet, it is absolutely crucial not to use them around sunrise. Accidently catching even a glimpse of the rising Sun through binoculars can cause permanent eye damage.
A Challenge for Southern Hemisphere Observers
This particular elongation offers a much clearer sighting for the northern hemisphere. For skywatchers in the southern hemisphere, Mercury will be only marginally visible, presenting a significant challenge that demands a perfectly clear eastern horizon.
This event underscores Mercury's reputation as a tricky planet to observe. Its orbit close to the Sun means it is usually obscured by solar glare, making these periods of elongation—occurring only a few times a year—the prime time for catching a glimpse of our solar system's swiftest planet.
