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Blue Moon Friday 31 July 2015

July 27, 2015 2 comments

This Friday 31 July 2015 there is an event which happens only “once in a blue moon”. Literally. This month there is a Blue Moon.

The occurrence of a Blue Moon doesn’t mean that the Moon will in fact turn blue. Instead a Blue Moon refers to a second Full Moon occurring within a fixed amount of time.

A full moon

A Full Moon, definitely not Blue

There are two widely accepted definitions of a Blue Moon: either it is an additional Full Moon within a season, or an additional Full Moon within a calendar month.

Moon Phases

Normally there are twelve Full Moons in a year, with one occurring every month. In fact the word “month” is derived from “Moon”. However the phases of the Moon don’t cooperate and divide the year perfectly into twelve with no left overs.

The Moon orbits the Earth every 27.32166 days, known as a sidereal month. As it does so we see different fractions of the lit half of the Moon, creating different phases. However during these 27.32166 days the Earth also orbits the Sun, and so the rate at which the phases change and repeat themselves is slowed down. Looking at the Moon from down here on Earth we see the pattern of phases repeating every 29.53059 days, known as a synodic month.

This is roughly one calendar month, but not exactly. It’s because of this “not exactly” that we don’t get a round number of Full Moons occurring every year, and don’t get exactly one occurring every calendar month.

In fact there are 12.37 Full Moons every year, and for this reason, every so often, we get 13 Full Moons in a year, which means an extra one in a season or in a calendar month.

The Maine Farmers’ Almanac Blue Moon (Type 1)

The original definition of the Blue Moon came from the Maine Farmers’ Almanac which defined a Blue Moon as the third Full Moon within a quarter-year season that has four Full Moons. Confused? You’re not alone. Normally a quarter-year season will have three Full Moons in it, as normally there are 12 Full Moons in a year. But due to that extra Full Moon that we sometimes get, every so often there are 13 Full Moons in a year. This extra Full Moon will occur in one specific season, and in that season the third of the four Full Moons is known as the Blue Moon.

Additional confusion arises due to the fact that the Maine Farmers’ Almanac uses a different definition of a season from the one astronomers use. Astronomers define the start and end points of the four seasons by the position of the Sun in the sky, or put another way the position of the Earth in its orbit. Because the Earth moves at different speeds at different points in its orbit the astronomical seasons are different lengths. Agricultural seasons in the Maine Farmers’ Almanac were all the same length.

This leads to the situation where a Blue Moon (as defined by the Maine Farmers’ Almanac) might occur in an agricultural season but not within an astronomical season. In order to avoid this additional confusion, seasonal Blue Moons are calculated with respect to the astronomical seasons these days.

For decades this definition of a Blue Moon held and was the only one. However now we have an alternative definition, thanks to a mistake in a prominent astronomy magazine.

The Sky and Telescope Blue Moon (Type 2)

In 1946 the astronomy magazine Sky and Telescope published an article by James Hugh Pruett in which he mistakenly interpreted the Maine Farmers’ Almanac. He correctly stated that due to the 12.37 Full Moons per year, we get an extra (thirteenth) Full Moon in seven years out of every 19. He then went on to state that the extra Full Moon that occurs in these seven years must occur in a specific month (correct) and that the second Full Moon in a calendar month is known as the Blue Moon (incorrect, according to the original definition).

Despite the fact that this definition of a Blue Moon was a mistake at the time, it was widely adopted, probably in large part due to its relative simplicity, and is the one that most people use these days.

This Month’s Blue Moon

This Friday’s Blue Moon is an example of a Type 2 Blue Moon, the second Full Moon within a calendar month (July 2015). The first Full Moon this month occurred on 2 July, leaving ample time for the second Full Moon to sneak in at the end of the month, on Friday 31 July 2015.

A Type 2 Blue Moon occurs on average once every 2.7 years. Most type 2 Blue Moons occur within months of 31 days, but they can occur in 30-day months. Because February is only 28 or 29 days long (shorter than the 29.53059 days of the synodic month) February can never have a Blue Moon (jn fact sometimes February has no Full Moons in it at all! The last time this happened was February 1999; the next time it will happen is February 2018).

Within any given century you can expect 37 Blue Moons, around 33 of which will occur in a 31-day month, and around seven of which will occur in a 30-day month.

Future Blue Moons

After this week’s Blue Moon the next one won’t occur until 2018, but then we get two that year! The first occurs on 31 January 2018 (Full Moons on 2 and 31 January 2018) and the second on 31 March 2018 (Full Moons on 2 and 31 March 2018).

After that we have to wait until 31 October 2020.

The next Blue Moon to occur in a 30-day month happens on 30 September 2031.

The Lowest Full Moon of the Year

Tonight (actually around 0130 tomorrow morning) the Full Moon will reach its highest point due south, just an hour and a half after the eclipse ends. Despite being at its highest in the sky, you’ll still struggle to see it, as it is very low down. In fact the Full Moon nearest the Summer Solstice is the lowest Full Moon of the Year.

Full Moon by Luc Viatour http://www.lucnix.be

First, let’s begin with the definition of “Full Moon”. A Full Moon occurs when the Moon is diametrically opposite the Sun, as seen from the Earth. In this configuration, the entire lit hemisphere of the Moon’s surface is visible from Earth, which is what makes it “Full”. There is an actual instant of the exactly Full Moon, that is the exact instant that the Moon is directly opposite the Sun. Therefore when you see timings listed for the Full Moon they will usually include the exact time (hh:mm) that the Moon is 180° round from the Sun (we call this point opposition).

Here’s a list of the times of all Full Moons between June 2011 and June 2012:

Month Date of Full Moon
Time of Full Moon (UT)
June 2011 15 June 2014*
July 2011 15 July 0640*
August 2011 13 August 1857*
September 2011 12 September 0927*
October 2011 12 October 0206*
November 2011 10 November 2016
December 2011 10 December 1436
January 2012 09 January 0730
February 2012 07 February 2154
March 2012 08 March 0939
April 2012 06 April 1919*
May 2012 06 May 0335*
June 2012 04 June 1112*

* UK observers should add on one hour for BST

As you can see from this table, the instant of the Full Moon can occur at any time of day, even in the daytime when the Moon is below the horizon. So most often when we see a “Full Moon” in the sky it is not exactly full, it is a little bit less than full, being a few hours ahead or behind the instant of the Full Moon. I’ll refer to this with “” marks, to distinguish this from the instant of the Full Moon (they look virtually identical in the sky).

The Moon rises and sets, like the Sun does, rising towards the east and setting towards the west, reaching its highest point due south around midnight (although not exactly at midnight, just like the Sun does not usually reach its highest point exactly at noon). And like with the Sun the maximum distance above the horizon of the “Full Moon” varies over the year.

The Sun is at its highest due south around noon on the Summer Solstice (20 or 21 June) and at its lowest due south around noon on the Winter Solstice (21 or 22 Dec) (of course the Sun is often lower than this, as it rises and sets, but we’re talking here about the lowest high point at mid-day, i.e. the day of the year in which, when the Sun is at its highest point that day, that height is lowest…)

And because Full Moons occur when the Moon is directly opposite the Sun, you can imagine the Moon and Sun as sitting on either sides of a celestial see-saw: on the day when the Sun is highest in the middle of the day (in Summer), the Moon is at its lowest high point at midnight; and on the day when the Sun is at its lowest high point in the middle of the day (in Winter), the Moon is at its highest high point at midnight.

This means, in practical terms, that Summer “Full Moons” are always very low on the horizon, while Winter “Full Moons” can be very high overhead.

Here’s a table of the altitude of the “Full Moon” when due south. Remember the times in this table don’t match the exact time of the Full Moon, but instead have been chosen as the closest in time to that instant, and so have be labelled “Full Moon” (in quotes).

Month Date of
Full Moon
Time of
Full Moon (UT)
Time/Date of
“Full Moon” due S
Time from/since
instant of Full Moon
Altitude due S
(degrees)**
June 2011 15 June 2014* 0127BST 16 June 2011  +4h13m  10° 05′
July 2011 15 July 0640* 0012BST 15 July 2011  -7h28m  10° 24′
August 2011 13 August 1857* 0126BST 14 August 2011  +5h27m  19° 19′
September 2011 12 September 0927* 0049BST 12 September 2011  -9h38m  31° 49′
October 2011 12 October 0206* 0053BST 12 October 2011  -1h13m  44° 16′
November 2011 10 November 2016 0005GMT 11 November 2011  -3h49m  53° 24′
December 2011 10 December 1436 0030GMT 11 December 2011  +9h54m  56° 03′
January 2012 09 January 0730 0006GMT 09 January 2012  -7h24m  53° 36′
February 2012 07 February 2154 0031GMT 08 February 2012  +2h37m  43° 47′
March 2012 08 March 0939 0000GMT 08 March 2012  -9h39m  35° 37′
April 2012 06 April 1919* 0145BST 07 April 2012  +5h26m  21° 45′
May 2012 06 May 0335* 0102BST 06 May 2012  -3h33m  15° 20′
June 2012 04 June 1112* 0047BST 04 June 2012  -11h25m  11° 49′

* UK observers should add on one hour for BST
** The altitude here is based on my observing location in Glasgow, Scotland. You can find out how to work out how high these altitudes are here.

As you can see from this table, the highest “Full Moon” due S this year occurs at 0030 on 11 December 2011, when the Moon will be over 56° above the southern horizon (approximately the height of the midsummer mid-day Sun which culminates at 57°34′).

Compare this to the “Full Moon” this month, just after the eclipse, in the morning of 16 June, when the Moon barely grazes 10° above the horizon, and you can see just how low the midsummer Full Moon can be.

In fact the closeness of summer “Full Moons” to the horizon means that this is an ideal time of year to try and observe the Moon Illusion.

UPDATE: Here’s a very cool speeded up video of the Moon cycling through its phases, as see by the LRO spacecraft:

Easter Astronomy

April 22, 2011 4 comments

Being neither Christian nor Pagan I’m not inclined to pay much heed to Easter (except of course for the days off work and the chocolate eggs) but it does present an opportunity to talk about a little astronomy. Did you know, for example, that the date of Easter changes each year, and that it depends on the phase of the Moon?

The Venerable Bede

The Venerable Bede, a Northumbrian Monk, wrote, in his book “The Reckoning of Time” (725CE):

“The Sunday following the full Moon which falls on or after the [Spring] equinox will give the lawful Easter”

So that all seems fairly straight forward: wait till the Spring Equinox; wait till the next full Moon; wait till the next Sunday; and voila, Easter.

However complications arise because:

(a) the ecclesiastical date of the Spring Equinox does not always match the astronomical date; and

(b) the ecclesiastical date of the Full Moon does not always match the astronomical date.

Why is this?

Put simply the ecclesiastical Spring Equinox always falls on 21 March, whereas the astronomical Spring Equinox, that is the point in the year when the Sun passes from the southern to the northern celestial sphere, can occur on 19, 20 or 21 March (and occurs most often on 20 March), and so the starting date for the computation of Easter is often out.

Also the ecclesiastical date for the Full Moon takes no real account of the actual phase of the Moon, instead it refers to the 14th day of the lunar month in which the New Moon occurs between 8 March and 5 April, which can differ from the date of the Full Moon by up to three days. This is referred to as the Paschal Lunar Month, the word “Paschal” deriving from the Hebrew word for Passover, a Jewish festival commemorating the Exodus.

The table below shows how these two dates vary over the next decade, and how that effects the date of Easter.

 Year (A)
Date of Ecclesiastical Spring Equinox
(B)
Date of Astronomical Spring Equinox *
(C)
Date of Paschal Full Moon on or after A **
(D)
Date of Astronomical Full Moon on or after B
Date of Sunday on or after C (Easter) Date of Sunday on or after D
(Astronomical Easter)
 2011  21 March  20 March  18 April  18 April  24 April  24 April
 2012  21 March  20 March  8 April  6 April  8 April  8 April
 2013  21 March  20 March  30 March  27 March  31 March  31 March
 2014  21 March  20 March  15 April  15 April  20 April  20 April
 2015  21 March  20 March  4 April  4 April  5 April  5 April
 2016  21 March  20 March  24 March  23 March  27 March  27 March
 2017  21 March  20 March  12 April  11 April  16 April  16 April
 2018  21 March  20 March  1 April  31 March  1 April  1 April
 2019  21 March  20 March  19 April  21 March  21 April  24 March
 2020  21 March  20 March  9 April  8 April  12 April  12 April

* Despite these dates being all the same, the date of the astronomical Vernal Equinox can vary between 19, 20 and 21 March, but occurs most often on 20 March. Over the next 100 years it will fall on 19 March only 4 times, and on 21 March only 20 times.

** The dates of the Lunar Months (there are 13 Lunar Months in one Solar Year) repeat on a 19 year cycle, called the Metonic Cycle. 2011 in the 17th year in that cycle; in 2014 we’re back to year 1. The dates for the Paschal New Moon can be found in calculation tables.

So you can see that, despite the fact that the Ecclesiastical Spring Equinox date doesn’t match the Astronomical Spring Equinox date on any year over the next decade (and only matches it in 20 out of the next 100 years), plus the fact that the date of the Paschal Full Moon often doesn’t match the date of the astronomical Full Moon, the dates of Easter calculated using the Ecclesiastical and the Astronomical methods only differ on one year in the next decade – 2019CE, when those who are so inclined will celebrate Easter on 21 April, but when, according to Bede, the Sunday after the full Moon after the equinox actually occurs on 24 March!

Maybe I’ve got too much time on my hands…

[Please note that throughout this post I have calculated and referred to the dates of Gregorian Easter, that celebrated by the western Christian churches. Eastern Orthodox Christians calculate Easter using the Julian calendar, which is currently 13 days ahead of the Gregorian calendar, i.e. on 21 March (the Spring Equinox) the Gregorian calendar reads 3 April.]

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