Shine On Harvest Moon

The Moon is now waning after the Blue Moon of 21 August, and the next full Moon is on 19 September - just 3 days before the autumnal equinox. And the full Moon nearest to the equinox is known as the Harvest Moon.

Next year will be interesting - the equinox is at 3.29am (Central European Time/British Summer Time) on 23 September, while there are full Moons at 2.38 am on 9 September and 11.51am on 8 October (which coincides with a total lunar eclipse, which won't be visible from the United Kingdom). The September one is 14 days 0 hours 51 minutes before the equinox and the October one is 15 days 8 hours 22 minutes after the equinox, so the September one is an early Harvest Moon.

In 2015 it is a bit simpler - equinox at 9.21 am (CET/BST) on 23 September, and the Harvest Moon at 3.50am on 28 September, which coincides with a total lunar eclipse which is visible from the United Kingdom and is worth getting up early on a Monday morning for.

There is one thing that the Harvest Moon is well known for, which is that Moonrise is nearly the same each night around it. If we look at the week centred on this we will see what happens (times in CET/BST):

Date	Moonrise	Later by	Moonset	Later by	Time Above Horizon	Longer by
September 15/16	4.56pm	39 mins	2.54am	1 hr 16 mins	9 hrs 58 mins	37 mins
September 16/17	5.30pm	34 mins	4.12am	1 hr 18 mins	10 hrs 42 mins	44 mins
September 17/18	6.00pm	30 mins	5.29am	1 hr 17 mins	11 hrs 29 mins	47 mins
September 18/19	6.27pm	27 mins	6.46am	1 hr 17 mins	12 hrs 19 mins	50 mins
September 19/20	6.53pm	26 mins	8.01am	1 hr 15 mins	13 hrs 8 mins	49 mins
September 20/21	7.19pm	26 mins	9.13am	1 hr 12 mins	13 hrs 54 mins	46 mins
September 21/22	7.47pm	28 mins	10.23am	1 hr 10 mins	14 hrs 36 mins	42 mins
September 22/23	8.18pm	31 mins	11.29am	1 hr 6 mins	15 hrs 11 mins	35 mins

As we can see, the amount of time the Moon is above the horizon is increasing, and the time of moonset is getting later at a faster rate than that of moonrise.

Around the time of the Harvest Moon, the Moon's declination is increasing. The Harvest Moon itself will normally happen in Aquarius (all images from Heavens Above)

or in Pisces:

. Sometimes, to oonfuse the astrologers, it's in Cetus:

When I looked at how the nights were drawing in, I noted that the greater an object's declination, the longer it spends above the horizon. And this is what we see around Harvest Moon - the Moon is moving northwards, so is spemding longer above the horizon. It is still at its highest between 45 and 50 minutes later each night, but the two effects means that the time between moonrises is decreased and that between moonsets is increased.

Can this lengthening of the time the Moon is above the horizon ever become big enough to push moonrise earlier? Not from Southampton, but look what happens if we go to Tromsø, at a latitude of 69 degress 40 minutes - so above the Arctic Circle. The times are Eastern European Time, which is 1 hour ahead of CET/BST:

Date	Moonrise	Later by	Moonset	Later by	Time Above Horizon	Longer by
September 15/16	6.21pm	1 min	0.47am	1 hr 57 mins	6 hrs 26 mins	1hr 56 mins
September 16/17	6.20pm	minus 1 min	2.42am	1 hr 55 mins	8 hrs 22 mins	1hr 56 mins
September 17/18	6.18pm	minus 2 mins	4.32am	1 hr 50 mins	10 hrs 14 mins	1 hr 52 mins
September 18/19	6.16pm	minus 2 mins	6.22am	1 hr 50 mins	12 hrs 6 mins	1 hr 52 mins
September 19/20	6.14pm	minus 2 mins	8.08am	1 hr 46 mins	13 hrs 54 mins	1 hr 48 mins
September 20/21	6.12pm	minus 2 mins	9.52am	1 hr 44 mins	15 hrs 40 mins	1 hr 46 mins
September 21/22	6.11pm	minus 1 min	11.35am	1 hr 43 mins	17 hrs 24 mins	1 hr 44 mins
September 22/23	6.10pm	minus 1 min	1.16pm	1 hr 41 mins	19 hrs 6 mins	1 hr 42 mins

At higher northern latitudes, there is a greater difference in time between the horizon for a shift of 1 degree in declination, compared to lower latitudes. And so we see here that it is possible, around the time of Harvest Moon, for moonrise to get slightly earlier each night.

We are actually around the point where this effect is least pronounced. I noted that the Moon can be above or below the ecliptic, and that currently when the Moon is at its furthest north, it is below the ecliptic - hence conversely, when it is at its furthest south, it is above the ecliptic. This means that it has a smaller-than-average declination range. Hence when it is passing through the Aquarus/Pisces area, its declination is increasing at a slower-than-average rate, and as a consequence the time it is above the horizon is increasing at a slower-than-average rate.

If we go forward 11 years to 2024, then we have the opposite effect. And if we look at the week around the Harvest Moon, which is at 4.34am (CET/BST) on 18 September, then we get:

Date	Moonrise	Later by	Moonset	Later by	Time Above Horizon	Longer by
September 14/15	6.15pm	31 mins	2.24am	1 hr 27 mins	8 hrs 9 mins	56 mins
September 15/16	6.38pm	23 mins	3.56am	1 hr 32 mins	9 hrs 18 mins	1 hr 9 mins
September 16/17	6.56pm	18 mins	5.28am	1 hr 32 mins	10 hrs 32 mins	1 hr 14 mins
September 17/18	7.11pm	15 mins	6.59am	1 hr 31 mins	11 hrs 48 mins	1 hr 16 mins
September 18/19	7.24pm	13 mins	8.31am	1 hr 32 mins	13 hrs 7 mins	1 hr 19 mins
September 19/20	7.39pm	15 mins	10.03am	1 hr 32 mins	14 hrs 24 mins	1 hr 17 mins
September 20/21	7.55pm	16 mins	11.35am	1 hr 32 mins	15 hrs 40 mins	1 hr 16 mins
September 21/22	8.16pm	21 mins	1.06pm	1 hr 31 mins	16 hrs 50 mins	1 hr 10 mins

As we can see, with the Moon's declination increasing at a faster rate than in 2013, the gaps between moonrise are smaller. What about Tromsø?:

Date	Moonrise	Later by	Moonset	Later by	Time Above Horizon	Longer by
September 14/15	Below horizon
September 15/16	8.54pm	N/A	1.02am	N/A	4 hrs 8 mins	N/A
September 16/17	7.57pm	minus 57 mins	3.53am	2 hrs 51 mins	7 hrs 56 mins	3 hrs 48 mins
September 17/18	7.22pm	minus 35 mins	6.18am	2 hrs 25 mins	10 hrs 56 mins	3 hrs
September 18/19	6.53pm	minus 29 mins	8.41am	2 hrs 23 mins	13 hrs 48 mins	2 hrs 52 mins
September 19/20	6.22pm	minus 31 mins	11.13am	2 hrs 32 mins	16 hrs 51 mins	3 hrs 3 mins
September 20/21	5.42pm	minus 40 mins	3.00pm	3 hrs 47 mins	21 hrs 18 mins	4 hr 27 mins
September 21/22	3.52pm	minus 1 hr 50 mins	7.50pm on 28 September	6 days 4 hrs 50 mins	7 days 3 hrs 58 mins	6 days 6 hrs 40 mins

So, at these times, this is an interesting effect we see at these latitudes. The Moon spends nearly a week too far south to be seen, and then it appears, but rises earlier each evening. Then there is about a week when it is circumpolar before moving southwards, this time with moonset being earlier each night.