Today is the first day of spring in the northern hemisphere! At 0430UT (GMT) this morning, 20 March 2016, the Earth’s axis of rotation went momentarily side-on to the Sun. For the past six months the Earth’s northern hemisphere has be angled away from the Sun, and now we’re angled towards it. Lots more info about equinoxes and equiluxes in a previous post.
As most people take delight in the lengthening daylight hours spare a thought for the amateur astronomers who, in a few weeks time, will be packing away their scopes for the summer, eagerly waiting for the darkening nights later in the year.
At 2249 GMT on 2 January 2016 the Earth reaches perihelion, its closest approach to the Sun this year.
If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.
The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun.
The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis.
Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).
The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.
This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.
The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.
You might have seen that there will be a Full Moon this Christmas Day, this first time this has happened since 1977.
It’s been described on social media as the “perfect” Full Moon. Quite what that means I’m not sure, but there is one factor that makes this month’s Full Moon stand out against the others; it will be this year’s highest Full Moon. But this is true of the Full Moon that happens nearest the winter solstice every year, so there’s nothing particularly “perfect” about this one.
A Full Moon occurs when the Moon is opposite the Sun in our sky, and we see the entire lit hemisphere of the Moon, making it appear full and round.
After it’s full, the Moon will appear to shrink to a gibbous moon (less than full but more the half), a half moon (called the last quarter moon), then a crescent, then a new moon, before growing again through crescent, half, gibbous and back to full. This pattern repeats itself every 29 days, which is approximately one month.
But because it isn’t exactly a month, and because our months vary in length between 28 and 31 days, the Full Moon doesn’t occur on the same numbered day every month, it drifts, seemingly at random.
For example the last decade’s December Full Moons happened on 15, 5, 24, 12, 2 AND 31*, 21, 12, 28, 17, 6 December.
(* In 2009 there were two full moons in December; the second is colloquially known as a blue moon.)
As the 29-day pattern of phases drifts around the 31 days in December it’s not surprising that the occurrence of Christmas Day full moons is around once every thirty years.
The last time this happened was 38 years ago in 1977; the next time it will happen is 19 years in the future, in 2034; an average of every 29 years.
Observing the Moon
If Santa was generous enough to bring you a telescope for Christmas, the Moon is one of the very best places to start. But you don’t want to observe it when it’s full.
This is mainly due to the overall lack of contrast and shadow on the Moon’s surface when it’s lit head-on by the Sun.
The best views of the Moon can be had when you can clearly see the dividing line between light and dark. This line is called the terminator, and when you observe it through a telescope the Moon appears in 3D, with shadows inside craters and beside mountains that really give you a feel for the structure of the Moon’s surface.
In addition the Full Moon is so bright that it drowns out the light of lots of faint astronomical objects, meaning that you would ideally wait till the moon is New, or a think crescent, before venturing out with your telescope to hunt for galaxies and nebulae.
So for stargazers this Christmas Day Full Moon is very far from “perfect”…
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.
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.
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.
Today, 23 September 2014, marks the moment of the Autumn Equinox. At 0229 UT (0329 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.
The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:
- Astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
- Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.
- So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0625BST in Glasgow (where I am) and will end this evening around 1950BST, giving me approx. 13.5 hours of “daylight”. (Londoners will have from about 0615 until 1930BST, or approx. 13.25 hours of “daylight”).
The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).
The northern hemisphere summer solstice occurs today, 21 June 2014 at 1051 UT (which is actually 1151 BST in the UK).
But surely the summer solstice is just the longest day. How can it “occur” at a specific instant?
That’s because we astronomers define the summer solstice as the instant when the Sun gets to its furthest north above the celestial equator. Or to put it another way, the instant when the north pole of the Earth is tilted towards the Sun as far as it can.
And this happens at exactly 1051 UT on 21 June 2014.
It’s important to remember though that while we are in the midst of summer, the southern hemisphere are experiencing their winter solstice, and their shortest day.
And how much longer is our “longest day”? In Glasgow, my home town, the Sun will be above the horizon for 17h35m12s today (21 June), a full one second longer than yesterday, and six seconds longer than tomorrow.
The northern hemisphere winter solstice occurs on 21 December 2013, at 1711 GMT. At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2013.
The actual day of the winter solstice – in this case 21 December 2013 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.