There’s a very thin Moon visible in the evening sky tomorrow, 8 May. You’ve probably seen thin lunar crescents before.
They’re especially prevalent at this time of year in the evening sky after sunset, a thin fingernail of lunar dayside hanging in the dusk twilight.
Beyond about 2% or 3% illumination, a crescent Moon isn't too hard to see, unless there’s haze around or they’re low above the horizon.
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For example, you can see a 3.7%-lit crescent Moon on the evening of 9 May, visible low above the west-northwest horizon after sunset.
At this percentage of illumination, although it might not be blindingly obvious at first, it can be seen and locked onto relatively easily, given clear skies.
This is much easier, of course, if your horizon is flat in the direction of the Moon, such a situation allowing the crescent to appear clearer at lower altitude as the sky darkens.
The thin crescent on 8 May is a different beast altogether.
There’s a threshold to when you can see a thin crescent either side of the Sun.
Known as the Danjon limit, this takes into account the effects of the rough edge of the Moon and how
it prevents the crescent from smoothly progressing from minimum illumination through to ultra-thin crescents.
Dates and timings for May's thin Moons
Getting a very thin Moon in the right place at the right time is a matter of luck, but on 8 May this happens.
On this occasion, from the centre of the UK, the Sun sets around 20:52 BST (19:52 UT), a 0.6%-lit, 15.6-hours-old Moon sitting just shy of 9° to the northeast of the Sun’s position.
The Moon sets 68 minutes later at 22:00 BST (21:00 UT) – be aware, though, that the times of sunset and moonset vary slightly depending on your location within the UK.
Use sunset as the starting time of the event rather than our stated clock times.
Twenty minutes after the Sun has set, it’s safe to use binoculars to start hunting.
Look for bright Jupiter just 1.5° above the northwest horizon 20 minutes after the Sun has gone down.
Despite shining at mag. –1.8, low altitude will make Jupiter hard to see. At the same time, the Moon will lie 5° above the horizon and will be just as hard to spot.
Thirty minutes after sunset and Jupiter will be 0.5° up. Aldebaran may become visible at this time, a mag. +0.8 star 7° up.
This gives a good measure to the Moon’s position, which will be 4° above the horizon at 30 minutes after sunset.
Forty minutes in and Jupiter will have set. Aldebaran will now appear 6° up, the Moon just 3° up.
This is probably the optimum time to see the Moon, a balance between atmospheric attenuation at low altitude and the darkening twilight.
Fifty minutes in and the Moon will be 1.8° up and becoming difficult to see.
This guide appeared in the May 2024 issue of BBC Sky at Night Magazine.
