Wednesday, 13 August 2014

The Moonlit Perseids

This morning, I went out at 1/2 past 4, and the sky looked like (image from Heavens Above):

My aim was to see some of the Perseid meteors, which were at maximum last night. In about 20 minutes out there, I just couldn't count the number I saw. It was zilch, zero, none...

There were two problems. First of all, it was getting light. Secondly, the Moon makes the sky bright. If we look at the lunar phases around the Perseid maximum for this year and the next two we have (with the times being in British Summer Time/Central European Time):

Year New Moon First Quarter Full Moon Last Quarter
2014 3.13pm 25 August 1.50am 4 August 7.09pm 10 August 1.26pm 17 August
2015 3.53pm 14 August 8.31pm 22 August 11.43am 31 July 3.03am 7 August
2016 9.45pm 2 August 9.21pm 10 August 10.27am 18 August 4.41am 25 August

As we can see, this year the Perseid maximum was between Full Moon and Last Quarter - when the Moon will rise after sunset and still be up at sunrise. Next year it'll be just before the New Moon. In 2016 it will be between First Quarter and Full Moon - when the Moon will rise before sunset and have set by sunrise. As you can see, the area where the Perseids tend to be seen (such as Cassiopeia, Auriga, Aries, Triangulum, Pegasus, Andromeda, and Perseus itself) is higher in the morning sky, so in 2016 there would be a window of darkness between moonset and when it gets light.

There is an interesting cycle with the Moon. Some time back, I noted the synodic month - the period of 29.530589 days, the average time from New Moon to New Moon.

Now consider 3 calendar years. If they don't contain a leap year, then they have 1,095 days. If they do contain a leap year, then they have 1,096 days. 37 Synodic months is 1,092.631793 days - so this is 2 days 8 hours 50 minutes (or 3 days 8 hours 50 minutes) short of 3 years. So, if you have a New/Full Moon or First/Last Quarter on one day one year, then 3 years later on, you should expect it re-occur 2 to 4 days earlier. We see this as this year the New Moon is 25 August, and in 2017 the Great North American Solar Eclipse is on 21 August.

When it comes to meteors, ideally you need dark skies in the morning (most meteor showers come from parts of the sky that are higher in the morning than the evening). This means you the Moon to be setting during the night - the earlier after sunset the better. For that you need the Moon to be waxing - that period between New Moon and Full Moon (the 2016 conditions) - or for it to be so close to New that it doesn't get very high in the dawn sky (the 2015 conditions). As a rule of thumb, expect lunar conditions to repeat 3 years later - which is an OK rule of thumb as long as you don't use it for long periods. So, we should expect the 2017 Perseids to be affected by moonlight, the 2018 Perseids to be around New Moon and the 2019 Perseids to be between First Quarter and Full Moon.

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