Perihelion 2016

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.

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Summer Solstice 2013

The northern hemisphere summer solstice occurs today, 21 June 2013 at 0504 UT (which is actually 0604 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 0504 UT on 21 June 2013.

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 Return of Darkness

With summer coming to an end in the British Isles we start the return to the dark skies of autumn and winter. Depending on where you are in the country you will have been without truly dark skies for many weeks, maybe even months, as summer evening twilight lasts throughout the night during the summer.

This all-night-long twilight is almost gone throughout the UK, indeed anywhere on the mainland UK can see astronomically dark skies around 1am at the moment. Only the furthest north outpost of the British Isles still doesn’t have that opportunity.

On the island of Unst, the furthest north of the Shetland islands, lies the UK’s furthest-north town, Skaw, at 60°49’N and 00°47’W. This tiny village will see astronomical darkness return at 0043 on 24 August, lasting only 46 minutes until at 0129 the sun’s light begins to creep into the sky again.

Midsummer sunset (at 2241) from Unst. Image Credit Brydon Thomason, Shetland Nature Ltd.

The last time that astronomical darkness was seen at Skaw was on 18 April, over four months ago! Indeed this settlement is so far north that between around 13 and 29 June each year they never get out of civil twilight, meaning that the sky’s bright all night long!

Compare this with the furthest south town in the British Isles, Saint Clement in Jersey, in the Channel Islands. Astronomical darkness returned to Saint Clement on 4 July this year, having been absent since 8 June; only four weeks without true darkness!

Such is the effect of differences in latitude that these two settlements, separated by 1299 km, have such hugely different seasonal swings between summer and winter.

 

 

Summer Solstice 2012

The northern hemisphere summer solstice occurs today, 20 June 2012 at 2309 UT (which is actually tomorrow in the UK, 21 June 2012 at 0009 BST).

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 2309 UT on 20 June 2012.

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 17h35m13s today and tomorrow (20 and 21 June), a full seven seconds longer than yesterday, and eight seconds longer than 22 June.

Perihelion 2012

At around 0100 GMT on 5 January 2012 the Earth will be at 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 (5 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.

Twice Around the Sun

Today is my little boy’s second birthday. He was born at 0940 on Wednesday 17 June 2009, and so today is the day that the Earth completes two orbits of the Sun since his birth.

However at 0940 today the Earth will not quite have completed its two orbits of the Sun; it will have a little bit further to go. Around 1.3 million km further to go to be precise, and it’ll cover that distance in 12h 18m 20s…

Happy Second Birthday!

Our calendar year is divided up into 365 days, the closest whole number of days to the actually time it takes to Earth to orbit the Sun, called the sidereal year, which is equal to 365.256363004 days.

Therefore the year (the time it takes the Earth to orbit the Sun) isn’t exactly 365 days long, and so every four years we add on one extra day in February to get a 366 leap year to make up for the difference.

If we didn’t do this, and instead insisted that the year was always 365 days long, the seasons would begin to drift until midsummer’s day (21 June on our calendars) would fall in the middle of winter within 753 years, a problem for our descendents. *

So, since my son was born there have been 2 x 365 days marked off on our calendar (730 days), but in order for the Earth to make two complete orbits of the Sun we have to wait 2 x 365.256363004 = 730.512726008 days, or an extra 0.512726008 days, which equals 12h 18m 20s.

My son was born at 0940, and so he will be two sidereal years old at 2158 this evening.

If I was interested in the instant when the Sun returned to exactly the same point within the cycle of the seasons I would have to carry out the same calculation except this time using the tropical year of 365.24219 days. In this case two tropical years equals 730.48438 days, or an extra 0.48438 days, which equals 11h 37m 30s.

My son will be two tropical years old at 2117 this evening.

Happy birthday, little spaceman!

* We have to calculate the drift based on the tropical year, the length of time it takes for the Sun to return to the same place in the cycle of the seasons, which is equal to 365.24219 days. A basic 365 day year differs from this by 5h 48m 45s, which would result in the seasons drifting so that midsummer’s day falls in midwinter in 753 years.

Even the 365.25 day leap year differs from this tropical year by 11m 15s, resulting in a drift which happens much more slowly than 753 years, but it still happens, taking 23,383 years for midsummer’s day to fall in the middle of winter. To correct for this we fine-tune our leap year formula so that we do not add in an extra day in a year which is divisible by 100 unless it is also divisible by 400!

This makes the average length of a year 365.2425 days, which still differs from the tropical year by 26.8 seconds. This barely matters, and no further corrections to our calendar are needed since the seasonal drift in this case will take over 500,000 years to happen, and by that time the Earth’s year will have changed in length anyway!

Year	Length of Year (days)	Difference from Tropical Year (seconds)
Tropical Year	365.24219	0
Basic Year	365	20,925s (5h 48m 45s)
Simple Leap Year	365.25	675s (11m 15s)
Fine-tuned Leap Year	365.2425	26.8s

101 Darkless Nights

As an astronomer in the depths of a Scottish summer I have a long time to wait until darkness falls again. Between 5th May and 8th August this year – for 101 nights – the sky never becomes truly dark.

In order to get a truly dark sky you need to wait for the end of astronomical twilight, when the Sun sinks lower than 18 degrees below the horizon. However in particularly northern (or southern) latitudes there is a stretch of time during the summer when the Sun never gets this low, and the closer to the poles you are the longer this run of darkless nights can be.

So astronomers often go in to hibernation during these summer months, unless they want to observe or image those astronomical objects that don’t require true darkness: the moon, say, or the planets.

The Sun will next dip 18 degrees below the horizon (as seen from central Scotland) on 9th August around 1am, and then only for half an hour. As the days go on though it dips lower and for longer each night, and the hours of stargazing stretch until around the winter solstice we will have close to 12.5 hours of absolute darkness.

Plenty of time then to stock up before another long darkless summer.

Bye-bye Orion…

Tonight is a symbolic and poignant one for me here in Glasgow; tonight Orion’s right foot Rigel sets at 2232, by which point astronomical twilight won’t even have ended.

Screen capture from the excellent Starwalk app for iPhone

This means that Orion is setting into the glow of the Sun in the west, and very soon won’t be visible at all. By the end of April Orion will be setting by 2100, only minutes after the Sun, so will be invisible until it appears next winter.

These dates will differ somewhat for you depending on where you are: if you’re further north than Glasgow Orion will already have begun to vanish in the longer days, and if you’re further south it will linger a bit, but to all intents and purposes that’s it for Orion till winter.

So take the opportunity to go and see Orion the Hunter while you can, before he vanishes into the glare of the Sun.