Moon half full

Moon half full

By Ceri Rees

We thought we would republish this blog, which we wrote last year after being contacted by Professor Roger Griffin,Professor Emeritus of Observational Astronomy and a leading researcher into Stellar Radial Velocities and Binary-Star Orbit Determination at the Institute of Astronomy, Cambridge University.

Is your moon half full or half empty?
At the moment, night races seem to be in wax, orbiting around the running calendar in a reassuringly predictable fashion. But how much thought goes in to the cycle of the moon, when planning these events? When I started organising night races with my second cousin Ben three years ago, we didn’t pay much heed to the moon’s influence. Watching a train of runners disappear over the horizon on Dartmoor, was special enough. Then we received an email, which may have changed all that.
It was from another organiser, who likes to do things by the light of a full moon. He seemed to have a bit to say about the moon’s role in all of this and so we did some research.
He just happened to be a Professor Emeritus of Observational Astronomy and a leading researcher into Stellar Radial Velocities and Binary-Star Orbit Determination at the Institute of Astronomy, Cambridge University. Although now aged 79, Professor Roger Griffin also happens to be a keen runner and runs almost every day and is a member of the University’s Hare and Hounds. For several years now, he has staged a somewhat spontaneous Midnight Moonlit Meadows Run, which starts just as the moon is about to hit its highest point in the night’s sky, on the Meridian.
He even abandons his post to take part in the annual flit across Grantchester Meadows to the church, starting at 23.38pm. No head-torches are needed and the idea is to reach Grantchester Church, about 2.5 miles away, just in time to hear the clock strike midnight, before turning around and running for home. It is a spontaneous affair, only circular emailing participants a few hours before the event, to confirm proceedings.
No ordinary observer of the skies, Prof Griffin makes a majority of his observations on-site with his 36 inch telescope. More binary-star orbits have been determined by him than by any other observatory in the world. So when he contacted us to suggest we stage our run, two days before the full moon, we were all ears. We had hoped to stage our off road half marathon night race The Moonlit Flit, under the light of a full moon. This isn’t always possible, given the crowded race calendar.
A full moon near to the Winter Solstice, which falls on December 21, occupies the same place in the sky as the sun does at the summer solstice on June 21. So it goes high up in the sky, and is highest at midnight because when full, it is opposite to the sun in the ‘celestial sphere ‘. In open landscapes, this is enough light to run by.
Because the full moon is roughly ten times brighter than the half-phase, you can not afford to go too far away from full if you want to have enough light to see by.
At the same time you need to have it reasonably high in the sky, which really means waiting until quite late in the evening. While, midnight on the night of full moon is ideal for some people like Roger, it was not for us, as it would have significantly affected turnout.
A slight mix up with dates, meant that we didn’t quite get a full moon for The Moonlit Flit in March. But we now know that next year, we won’t have to wait until midnight in order to benefit from full moonlight (thankfully as this would have affected turn out considerably). If we started the run at around sunset we would not get much value, even from a full moon because it would practically be on the horizon.

The Professor explained: “The moon moves among the stars from right to left, and the amount that it moves between one night and the next is about equivalent to how far it moves (complete with the star field in which it is situated that night), just in the ordinary course of rising and setting as a result of the rotation of the Earth, in one hour. So the night previous to a full moon, the moon will rise about an hour before sunset, two nights previously it will rise two hours before, and so would then be at a reasonable altitude by the time the sky was dark, though not quite so bright as when it is exactly full.
“Then, you do not need to wait until it is exactly on the meridian, i.e. due south, where it reaches its highest altitude – it is nearly as high for the hour or two on either side of that time. On the actual night of Full Moon, it will be on the meridian very near midnight, but on the previous night it will reach the meridian around 11pm, and on the one before that 10pm; then, in view of its being nearly as good when it is an hour or two short of its highest point, you will appreciate that it could be satisfactorily bright by about 9pm on the night before it is Full and at 8pm on the night before *that*.
“Thus there is a possibility of scheduling a moonlight run as early in the night as 8pm if you choose the night next but one before the moon is full. But it will not be quite as good as the ones that I have sometimes arranged when the weather is good on the actual night nearest to full moon, when the conditions for bright moonlight are absolutely optimal.”

He went on to add these caveats: This only applies in the winter half of the year: In the summer the full moon is always low down in the sky, being in the part of the sky where the sun is in winter.
Although his assertion about the moon rising an hour later on successive nights is only very approximate, if you consult an appropriate calendar for sa, September, when the moon (because it is moving further north from night to night, corresponding to the movement of the sun from month to month around the time of the March equinox), it is still worth knowing.
Moonrise is only about half an hour later from one night to the next (the phenomenon of the ‘Harvest Moon’) whereas in March the change from night to night will be more than an hour.
Nearer to the winter solstice, the hour-a-day rule is close to the fact. Roger then calculated the times of the moonrise around the full moon in November (for latitude 52N), with the full moon on November 6th at 10pm. He found that for Nov 4, 5, 6 and so on, the rising times were 14:57,15:36,16:21,17:12,18:08,19:08 and 20:10.
So if you ever come across a bunch of runners, running across a field at midnight, close to the Solstice, you’ll know there is method to their madness.
As for Roger’s motivation: An act of defiance perhaps? He told us: “I still get a kick out of organising something that some of the young like to do but haven’t thought of organising for themselves, whereas I am old.”
Ceri and Ben organise the Wild Night Run Series in Devon, including The Wild Night Run (ten mile run on Dartmoor) on January 31 and The Moonlit Flit half marathon on March 21. (