Thin Moon spotting
BEST TIME TO SEE: 18 August from 90 minutes before sunrise; 19 August from just after sunset
In theory, as the Moon approaches its new phase in the morning sky, the reducing separation from the Sun should mean its illuminated phase gets thinner, reaching minimum at new Moon. Then, its phase should grow incrementally as another lunar cycle begins. In practice there is a period of invisibility either side of the Sun. Known as the ‘Danjon limit’, this defines when the lunar crescent should disappear and reappear either side of new Moon. The invisibility zone occurs because the Moon has a rough edge and this curtails the appearance of super thin Moons, limiting visibility to when the Moon is over 7˚ centre-to-centre from the Sun.
If it’s clear on the morning of 18 August, try spotting a thin 1%-lit waning crescent Moon, 11˚ from the Sun, just 22 hours before new Moon occurs. Note that moonrise occurs 90 minutes before sunrise on this date. For a trickier spot, try the evening crescent on 19 August. This one lies just under 10˚ from the Sun and sets 40 minutes after sunset.