Celestial spheres : The Bluebird Guide to Solstices and Equinoxes


I am often accused of being a far too city-centric person and am frequently on the receiving end of The Disapproving Eyebrow.  The Disapproving Eyebrow appears when accompanied with any of the following offhand comments : someone decides that I live in London because I refuse to live anywhere else, that I hate the countryside and that – no – I wouldn’t be interested in that Lake District beer quaffing and walking holiday because I am intrinsically wired against it / think The Wirral is a public house not an area / cannot achieve orgasm if taken beyond the M25 etc etc.   Well, for all the nay-sayers and those who think my world is too internally Londonified I say Ha!  I say Double Ha!  I say read on, as today we are going 149,600,600 km, which is longer than distance between John O’Groats and Clapham High Street.  Just.

Last Sunday the Autumn equinox occurred at 20:44hrs or, in UK parlance, shortly before Downton.   The evenings at this time of year are highly seductive, but ultimately a tease.  I say this because no sooner have you noticed the blissful violet radiance in the heavens above and prepared the candle and the glass of Venetian red in preparation for a swoon at the sexy sky in the mild temperature than it suddenly decides to switch off the lights and plunge you into deepest night.  This means that you are left, in dressing gown over optimistic spring-like outfit in deep darkness in your own garden, gesturing wildly in rage towards the night sky and blindly grabbing into black hellish nothingness for your wine glass.  And it’s only 7.30pm.  It’s like the promise of a beautiful stripper taking half of their clothes off, with a series of pouts and shoulder rolls to imply : “Feast your eyes, kiddo” and then suddenly deciding you won’t get any further than underwear, and he’s fucking off until next Spring.  You see?  Autumn.  Beautiful.  But a blatant seasonal tease.

My thoughts have turned celestial.  One night, you know the moment the planet turns.  It’s the moment you don’t want to drink white wine anymore.  It’s the moment you know a supper of fish and salad will no longer fortify.  It’s the moment you remember you haven’t got any tights.  If you’re self employed, you know its the moment you can no longer procrastinate when it comes to preparing this year’s books for your accountant’s meeting in mid-October.  Most of us though, don’t really rely on the planet to tell us this.  We just know this moment because Strictly starts.  However, I have become fascinated by the planetary activity that just sort of potters on in this magical planet of ours whilst we pettily provoke madnesses and silliness down here on terra firma,  keeping our small brains occupied with Sainsburys, booking waxes, cleaning bathrooms and topping up Oyster cards.  Exactly what is going on up there?  What has astronomy got in store for us through the year?   I have been researching this a little for you, and thought to produce a guide of equinoxes and solstices for you in layman’s terms and in Londoner’s terms.

Third Rock From The Sun

Yes, that is us.  We are on Earth, and we are 149,600,600km from the Sun.  That is the same distance as driving up and down the whole of the Finchley Road 21,371,515 times.  Earth is like an egg, that has been sat on for a bit and turned on its side.  The official word for this is ‘oblate spheroid’.  As it sits on its side, it is quite short.  It is bigger in width (equator) than it is in height (pole-to-pole).   It has a crust, a mantle and a core, and the top crust layer, on which we live,  varies in its thickness between 3 miles (Berwick Street to Essex Road) and 46 miles (Milton Keynes to London).

Ecliptic Plane

This is the path on which the Earth orbits the sun, together with the other planets in our solar system.  It is like an enormous M25 but as it is an imaginary plane I cannot tell you how much bigger than the M25 it really is.  But take it from me. It’s massive.  As imaginary lines go, the ecliptic is a big’un.  Unlike the M25 it is perfect, has no roadworks, you are unlikely to crash in a Vauxhall Astra on it, nor come across an enforced 40 mph three mile zone whilst on your way to a barmitzvah.   The ecliptic can be basically called the apparent path on the sun on the celestial sphere, or the imaginary line on which we move around the sun.

The Celestial WHAT?

The celestial sphere.  It’s a troublesome phrase, mainly because it sounds like a nightclub in Bushey.   Space is extremely large, and subsequently its tricky to place those things within it (moon!  stars!  planets!  Suzi Quatro!)  as they appear to be so remote.  The celestial sphere is best imagined as an enormous circle that we are all inside, and we see the stars and moon reflected on its domed underside.    Everything you can see in the night sky is reflected on what appears to be the inside of this enormous, imaginary circle.  The ecliptic plane also exists inside it.

Axis

Imagine you have a cocktail stick.  Perhaps it is a Thursday night and you are feeling jolly. You wish to make a martini.  You take a green olive (stoneless) and you spear the olive at a jaunty angle, going in from the top right corner and poking the cocktail stick out of the bottom left.   In fact, you are poking the cocktail stick through the olive at an angle of 23.5%.   That is the angle of the earth’s axis.  The distance from pole to pole is 12,416 miles, or the same distance as driving down the A406 from Chiswick to Beckton 786 times.   The planet spins at this angle, veering slightly to the right diagonal:

Earth Axis

Yes, but what has this to do with Autumn and why am I wearing thick tights?

Hang on. I’ll get there.  You have to know your ecliptic plane, axis and your celestial sphere to understand how the seasons work, because these things are called the celestial coordinate system.   The celestial sphere may be imaginary but it provides a framework for us to understand the earth’s axis and orbit, and also to understand how the sun shines.  This will go some way to describing why it is cold in winter and hot in summer.  You get seasons because of the axis I mentioned above. If the earth was perpendicular to the ecliptic plane, and did not have this axis,  it would not have the opportunity to experience seasons.

Celestial Sphere

Here is a picture of Earth as the small black circle in the middle.  The blue circle is the celestial sphere. See, I told you it was bigger than the London Orbital.  So, you can see from this picture how the South Celestial Pole is just a continued line directly into space towards the celestial sphere from the South Pole on Earth and how the North Celestial Pole is a continued line directly into space towards the celestial sphere from the North Pole on Earth.  Simples.

So, what is a Solstice?

A solstice occurs twice a year.  The Northern Solstice is when the Sun sits over the North Pole.  It also seems as if the sun is higher in the sky at this time.  This occurs on June 21st.  So, for us in the Northern Hemisphere it is our longest day of the year.  Note the Northern Solstice is not the Northern Line.  The Northern Line exists only several hundred feet beneath the Earth’s crust and has no up escalator at Tottenham Court Road due to Crossrail.

The Southern Solstice is when the sun sits over the South Pole and that’s when the lovely people in the southern hemisphere get to have their longest day. This occurs on approximately Dec 21st.  Most of us in the northern hemisphere do not notice it however, as it’s impossible to spot the shortest day in the dark when we are mainly drunk and wrapping Christmas presents.

Solstice comes from the Latin “sol” (sun) and “sistere” (to stand still).

Why can’t the Autumn and Spring ones be called Solstices too?  Why do we have to call them Equinoxes?

Because they aren’t the same thing.  Solstices indicate the extremes of night / day and cold / warm.  The sun is polar during solstices.  But on a equinox the sun appears to cross the equator.  It is always on its way between north and south poles and the equinox marks the half way route.  That’s why it happens in Spring and Autumn.  It’s genius!   The word Equinox is also Latin, translating roughly as “equal night”.  So during the Equinoxes we should have 12 hours each of dark and light respectively.   So, that’s March 21st and September 21st, although it differs by a day or two.

So, when is my smear test due?

I’m sorry.  The sun cannot tell you this.  But if it’s been more than 6 solstices you ought to get yourself to the GP.

Please return to The London Bluebird if you enjoyed today’s educational astronomical insights.  We update every two weeks so look forward to seeing you on Thursday 10th October

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