The Sky Guide Challenge
Can you spot the lunar city?
This month’s challenge is to track down and observe the clair-obscur feature known as Gruithuisen’s Lunar City. Despite its rather grandiose name, it’s actually quite tricky to find, not helped by the fact that it vanishes fast as the Sun sets on its ‘streets’.
A clair-obscur effect is a trick of the light that results in shadows appearing to create recognisable objects. They add an extra challenge to lunar observation as they only appear at certain times, when sunlight hits the Moon’s surface at the correct angle. The best-known are the Lunar X and Lunar V.
The time a clair-obscur effect will appear is given by the position of the Moon’s terminator rather than regular clock time; the Moon’s libration interferes with that. Consequently, clair-obscur effects are often described in terms of the Moon’s co-longitude, a figure identifying the location of the morning terminator on the lunar surface. The morning terminator is the one that leaves light in its wake as it sweeps across the Moon’s disc; the one that brings darkness is the evening terminator.
At first quarter the co-longitude is close to 0°; at full it’s near 90°; at last quarter (when the morning terminator is halfway across the far side of the Moon) it’s around 180°; and at new Moon it’s about 270°. Co-longitude cannot be precisely tied to a particular phase because of the distorting effects of libration. Gruithuisen’s Lunar City is visible approximately one day after last quarter, more specifically at lunar co-longitude 185°.
Freeware apps such as WinJUPOS (jupos. org/gh/download.htm) and Virtual Moon Atlas (www.ap-i.net/avl/en/start) provide co-longitude information. This month, co-longitude 185° occurs in the early afternoon on 2 October when the Moon is, annoyingly, below the UK’s horizon. It next occurs on 1 November, just after 02:00 UT.
It’s not a real city of course, but a lighting effect that makes a small section of the Moon appear to have linear ‘streets’ in a leaf-like pattern. To locate it, first find the giant craters Copernicus (93km) and Eratosthenes (59km). Imagine them forming one side of a large isosceles triangle. The other vertex lies further south, close to the morning terminator, marked by the much smaller crater Mösting (26km).
From Mösting, head back along the line towards Eratosthenes. About one fifth of the way you’ll find the heavily eroded Schröter (35km). Keep going towards Eratosthenes and the next small but identifiable crater is Schröter W (10km), which has a smaller crater, Schröter A (3km), inside it. The ‘streets’ of Gruithuisen’s city appear to radiate north from Schröter W.
The feature is named after Franz von Paula Gruithuisen who, from a time when it was believed the Moon was populated, made multiple observations of the ‘city’. He called it Wallwerk and was met with significant scepticism when he published his discovery of the city’s streets and buildings in 1824.
“A lighting effect makes a small section of the Moon appear to have streets”