Leap Year 2012

Thirty days hath September,

April, June, and November;

All the rest have thirty-one,

Save February, with twenty-eight days clear,

And twenty-nine each leap year.

This year is a leap year, when the month of February has 29 days in it, rather than the usual 28. The rhyme above is a mnemonic to help us remember the days in each month, but it doesn’t explain why we need leap years, and why they occur once every four years.

To understand the reason for leap years we have to look to astronomy, and in particular to the orbit of the Earth around the Sun. The Earth orbits round the Sun in 365.256363 days. This is a bit awkward as it means that the year cannot easily be divided into a whole number of days. If we round the year down to the nearest whole number of days we get 365 days in a year, which is indeed what we have in most calendar years.

So why not just leave it at that? Isn’t 365 days close enough to 365.256363? After all it’s 99.93% of the actual year, which is nearly 100% right, yes?

Actually; no. In ancient Egypt, where they lived with a calendar year of 365 days, the seasons began to drift at a rate of one day every four years. If we had stuck with the Egyptian calendar of 365 days every year then the longest day, which we take to fall on 21 June in most years would fall on 20 June four years later, then 19 June four years after that, until over the course of 730 years or so the longest day would occur when our calendars said it was the middle of winter.

Obviously something needed to be done to fix this problem. Enter Julius Caesar who, in 46BC, introduced what is known as the Julian Calendar. In this calendar Caeser recognised what Greek astronomers had long known; that the year is closer to 365¼ days long. They didn’t know that the Earth went round the Sun in 365¼ days, but they knew that the seasons repeated themselves on a 365¼ day cycle, and not a 365 day cycle as the Egyptians thought.

To account for this more accurate measure of the changing seasons, and to align the calendar better with the real world, Julius Caeser announced that every fourth year would have an extra day in it, to occur at the end of February. This would allow the calendar to keep in line with the real changing seasons, so that the longest day would always fall on the same day of the calendar.

But by 46BC the seasons had already drifted a lot; in fact the Roman calendar was about 80 days behind the actual seasons, so Caesar proclaimed that 46BC would have extra days in it, and be 445 days long, so that the calendars would be aligned on 1 January 45 BC, at which point the new calendar of leap years would begin.

The Romans didn’t call these leap years though; that name came along about 1400 years later. They were called “leap years” because the occurence of them every four years caused festive days (like Christmas), which usually advanced one weekday per year, to suddenly leap forward by two days. For example, Christmas Day in 2009 fell on a Friday, in 2010 on a Saturday, in 2011 on a Sunday, but this year, in 2012, it will leap forward to a Tuesday.

Not the Whole Story

Of course things are never that simple, are they? In fact the year is not 365¼ days long either, it’s 365.256363 days long if you measure it in terms of how long it takes the Earth to go round the Sun, or 365.242189 if you measure it in terms of how long it takes the Sun to return to the same part of the zodiac (which is indeed what we need to measure if we want to track the seasons).

We no longer have a Julian Calendar of 365 days each year with 366 every fourth leap year. Instead we have adopted the Gregorian Calendar where:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years.

So 1900CE wasn’t a leap year (nor was 1700 or 1800), event though it was due to be, but 2000CE was. This is to fine-tune our year to fit even better with the changing seasons. Without this slight tweak then even the Julian calendar would drift with the seasons, albeit not as drastically as the Egyptian fixed 365 day year.

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Venus, Jupiter and the Moon, February 26 2012

Tonight, just after sunset, Venus, Jupiter and a thin crescent Moon will line up in the evening sky. If you’ve got clear views towards the west it’s really worth a look.

Looking west after sunset on Sunday 26 Feb 2012

Thanks to the excellent Starwalk app for the above image.

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: