Thin Moon and Mercury
BEST TIME TO SEE: Morning of 24 October, from 07:00 BST (06:00 UT)
The visibility of the thinner crescent phases of the Moon is optimised around the equinoxes. Around the March equinox, it’s the waxing crescents that are well placed, the ecliptic making a steep angle with the western horizon at sunset. At this time of year, it’s the morning crescents that are optimally placed, the ecliptic making a steep angle with the eastern horizon around sunrise. The Moon never wanders that far from the ecliptic, its orbit being tilted by around 5˚ to the ecliptic plane.
On the morning of 24 October, the thin waxing crescent Moon rises above the eastern horizon 90 minutes before the Sun and will have a phase of 2%. It should be easy to spot as it rises, as the sky will still be relatively dark. Approximately 30 minutes after the Moon has appeared, Mercury will pop up above the same point on the horizon. At a magnitude of -1.0 and under brightening twilight skies, the planet should be easy to spot too. The pair will be 4.1˚ apart as they climb in altitude.
A sliver of a Moon followed by bright Mercury rise together just before sunrise
From the centre of the UK, the Moon will attain an altitude of 6˚ and Mercury 3˚ at 07:15 BST (06:15 UT).
This thin Moon should look particularly attractive as it should be showing the phenomenon of ‘earthshine’, where the night side of the Moon appears to glow gently against the surrounding sky. The effect is caused by sunlight reflecting off Earth, hitting the Moon and reflecting back towards Earth. If you were standing on the Moon’s Earth-facing side in the nighttime portion, the Earth would appear almost full and around four times larger than the Moon appears in our sky. Being more reflective than the Moon also helps Earth bring light to the Moon’s nearside night.