Lunar Circumhorizon Arc Visibility

The geometric conditions for seeing a lunar circumhorizon arc are the same as for a solar one. The moon being a lot fainter, and not always full, greatly reduces the number of practical opportunities of course. The visibility varies on a monthly basis rather than annual, making it harder to keep track of the best times. There is an interesting twist for high latitude observers because the moon can get about 5 degrees further north or south than the sun, extending the limit of visibilty. There is a catch but that is dealt with on the next page.

The diagram below shows how the visibility varied for London in 2006/2007. Note that the best times occur in winter when the moon is furthest north. The white cross marks the time the images on the previous page were taken.

The black areas are the times of complete darkness, the grey bands are when the moon is above the horizon. Yellow and orange are when a lunar circumhorizon arc is possible. White discs are full moon and black discs are new moon.

Timetables created with GraphDark

The amount of time when a lunar circumhorizon arc is possible also varies greatly with latitude. Timetables for the same set of locations are shown below for the months when the moon is furthest north.
Click on the small timetables for more detailed versions.

London, 52 degrees north

The time available is very short but at least it does get into the orange which is a more favourable altitude.

Madrid, 40 degrees north

As expected, significantly more opportunities.

Tenerife, 28 degrees north

The moon can now get too high for the arc to form.

Dakar, 15 degrees north

Not so obvious as with the sun but two periods of high altitude invisibility are now detectable.

Nairobi, 1 degree south

The twice monthly passage of the moon overhead for an equatorial location shows up quite well.