Lunar Circumhorizon Arc Visibility Cycles

Because the moon's orbit is tilted about five degrees from the ecliptic it can get further north than the sun. This can be an advantage for more northerly observers, however there is a downside. At other times the moon will be five degrees south of the ecliptic and not as far north as the sun at its highest. This occurs on a cycle of roughly 19 years. Although it is academic for lower latitude observers it means that northern Europe will have opportunities for lunar circumhorizon arcs in some years but not others.

This diagram shows how the visibility from London changes over the 30 year period 2000-2029.

Colour coded as per the previous page, yellow means a weak arc is possible but orange is better.

The change from year to year is not great so two years are shown per line.

The time available increases up to about 2005 and then declines again. No lunar circumhorizon arcs are possible from London between 2013 and 2017.

Things improve again to a peak around 2024 before declining again.

The chances of having suitable ice crystals in the right place at the right time during the few nights available are small. A clear lower atmosphere is also needed so sightings are going to be very rare.

Click on the diagram for a more detailed version.

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