Observing

The Summer Solstice 2014 and The Summer Night Sky

It’s been some time since I’ve blogged! I had hoped to blog a tad more regularly, but it’s becoming clear to me that it’ll take longer to get up to speed with regular blogging (with worthwhile content!).

So I’m still in a phase of extensive research and literature surveying for intended blogs on the future of Mars exploration, the changing landscape of space exploration and related sociological issue. When all of that is complete it will hopefully lead to some insightful blogs as there are many changes taking place right now in how space exploration is being planned and pursued, and on how people in general perceive it – and with MANY outstanding and issues too. Watch this space!

In the mean time, what a perfect opportunity to write a little about the summer solstice and the summer night sky from Ireland. As you may have heard on the RTE Radio 1 Marian Finucane Show on Sunday 22nd June, we chatted about the summer solstice, so I thought I’d post some details on the solstice from an astronomical point of view, and talk a little about what constellations are in the night sky over Ireland. A little rushed, but here goes:

Key Points:

Solstice:
– Moment / time each year when Sun reaches highest point in the sky
– When (2014): 11.51am on June 21st
– Height above the horizon from Ireland (Altitude): 60.5 degrees above the horizon
– For all points on Earth, including Ireland, the Sun reaches a different height in the sky for each and every day. In mid winter it reaches only about 13.5 degrees above the horizon (called the winter solstice), shortening our days; while in mid summer it reaches an altitude of 60.5 degrees (summer solstice), lengthening our days. Mid point between those extremes – at the spring equinox (March) and autumnal equinox (September) the Sun reaches an interim height (about 37 degrees) and we get approximately equal length days and nights
– Cause: Earth’s axis is tilted by 23.5 degrees, it is spinning and it is orbiting the Sun. So in summer Earth is tilted toward the Sun and we see it higher in the sky, in winter we’re tilted away from the Sun and we see it lower in the sky, and for every other day of the year the Sun reaches a height at noon between those extremes.
– To help visualize this, concentrate first on the extremes. So in summer we’re tilted 23.5 degrees towards the Sun meaning that when it rises into the sky it reaches a great height. Conversely in December we’re tilted 23.5 degrees away from the Sun so we see it correspondingly lower in the sky. On the equinoxes we’re neither tilted away nor toward the Sun so it reaches an interim height – and we get 12 hours day and night. Of course the Earth is always tilted at 23.5 degrees – that doesn’t change through the year; what we’re interested in here is the orientation of that tilt with respect to the Sun, and at the equinoxes the Earth is tilted at 23.5 degrees for sure, only neither towards nor away from the Sun.
– Each place on the Earth experiences the same affect but to differing amounts depending on its latitude on the globe; so every location in the northern hemisphere sees the Sun at its highest point in their sky on June 21st.
– To work out the maximum angle of the Sun above the horizon on the summer solstice at your latitude:
(90 degrees – your Latitude) + 23.5 degrees (that equals 60.5 degrees for Ireland)
– To work out the maximum angle of the Sun above the horizon at your latitude during the winter solstice:
(90 degrees – your Latitude) – 23.5 degrees (approximately 13.5 degrees for Ireland)

Explanation:
– The Earth orbits the Sun once a year and so the Sun appears to traverse the sky once a year (from west to east)
– We don’t see the Sun actually move from west to east throughout the year, rather we see it at about the same position at the same time each day, with the background stars (from our line of sight) changing accordingly throughout the year. To visualize this, imagine asking a friend to stand still while you walk around them keeping your eyes firmly fixed on them at all times. As you circle your friend they will stay in the centre of your field of view. However, what you see behind them will continuously change as you circle them. It’s like this for the Earth circling the Sun. We see the Sun at approximately the same place in the sky at the same time each day, but the stars or constellations behind the Sun change as the year passes by.
– So as Earth orbits Sun it appears to move in relation to the background stars. Ancient civilisations organised groups of stars in the sky into patterns called constellations; and the Sun passes through one constellation per month. Hence the 12 constellations it passes through are called the constellations of the zodiac, and are named: Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn and Aquarius. The path around the sky through the zodiac that the Sun appears to travel is called the ecliptic.
– But the tilt of the Earth has a significant effect on the Sun’s apparent yearly motion about the sky in a vertical sense (as in: higher in summer and lower in winter), and is the cause of us having longer days in summer and shorter days in winter.
– Because the Earths axis is tilted by 23.5 degrees the Sun moves around the sky once a year on a path that is tilted at an angle of 23.5 degrees to the celestial equator (the extension of the Earth’s equator into the sky that divides the sky into north and south). So the Sun’s yearly path is a great circle in the sky (called the ecliptic) that intersects the celestial equator at an angle of 23.5 degrees and so the Sun appears to travel from south to north and back again in a cyclical manner over its yearly path. The Sun is below the celestial equator for 6 months and so appears in southern part of the sky, and is above the equator for 6 months and appears in the northern sky. It is this south-to-north (and back again) apparent movement of the Sun in the sky over the course of a year that gives us longer and shorter days; where (for us in the north) the Sun reaches its furthest point north in the sky in summer, appearing higher in the sky and giving us longer days, while it reaches its furthers point south in winter and hence appears lower in the sky, giving us shorter day.
– The point when it traverses south to north – where the ecliptic intersects the celestial equator – is called spring equinox and gives us 12 hours of day and night (approximately). This is also called the First Point of Aries because when ancient civilisations discovered this phenomenon, the Sun appeared in the constellation of Aries as it travelled from south to north.
– Also, at the time of such celestial discovery about 2000 years ago, in mid-summer the Sun resided in the constellation of Cancer and also directly over head at a latitude of 23.5 degrees north of Earth’s equator. This is why that latitude is called the Tropic of Cancer. Similarly at mid winter in the north (mid summer in the south) the Sun resided in the constellation of Capricorn and also appeared directly overhead at a latitude of 23.5 degrees south of Earth’s equator and why that latitude is called the Tropic of Capricorn. Precession of the Earth’s axis over a 26,000 year period means that although Earth’s tilt remains about about 23 degrees, it’s orientation changes (think of a spinning top rapidly rotating at a tilt but the orientation of the tilt changing over time). As a result of this precession the constellation the Sun appears in at the equinoxes and solstices changes over time (by about 1 degree in the sky every 72 years) so that today the Sun no longer appears in Aries at the spring equinox but instead now appears in the constellation of Pisces; and similarly no longer appears in Cancer at the summer solstice and instead has moved into the constellation of Gemini at that time of the year.

Finally, Earth’s precession also means that, were it not for regular corrections we make to our calendar, mid-summer in the northern hemisphere would occur in December in about 13,000 years from now. To ensure we retain our existing calendar with mid-summer always in June, we add fractions of a second to our clocks every few years, nudging the calendar back into place to counteract the Earth’s precession.

The Summer Constellations

Of course, while we can’t directly see which constellation the Sun is in at any given moment (because that constellation is behind the Sun in the day time), as the Sun moves from constellation to constellation, it correspondingly affects which constellations we see at the night. In particular, those constellations on the far side of the sky to the Sun at any given moment will appear in the middle of the night.

This is why we see different constellations in the night sky during different times of the year. For example, the constellations of Taurus is visible in the southern sky in winter, but during the summer it is not visible at night because the Sun is actually in the constellation of Taurus so it is in the sky during the day.

So this gives us a lovely opportunity to ask what constellations are in the summer sky around the time of the summer solstice and though July? Though of course the skies are less dark than in winter (because the days are longer and the Sun does not set so far below the horizon at our latitude), the summer night sky from Ireland is nothing less than spectacular.

Firstly, traversing the sky from northeast towards southwest you see the faint white band of the Milky Way galaxy itself, of which we are a part. The Milky Way is a galaxy of perhaps 200 billion stars, each like our Sun, and the band of light is the combined starlight of all 200 billion of those stars.

The main feature of our summer skies is called the “Summer Triangle” made up of the three brightest stars you can see as you look overhead and south: the brilliant blue-white star Vega (in the constellation Lyra) almost directly over head at midnight through July, Deneb the strong white star in the magnificent constellation of Cygnus the Swan (also known as the Northern Cross) and the white-yellow star Altair furthest south in the constellation of Aquila. Altair is of particular note because, at just 16 light years distance, it is one of our closest cosmic neighbours (and is the star to which the gallant crew travelled in the iconic 1950’s film “Forbidden Planet”).

The constellation of Cygnus the Swan is also of particular interest. To find it, look for a large cross or crucifix outline of seven stars almost directly over head during July, of which the brightest star is Deneb. Cygnus lies within the rich star fields of the Milky Way, and viewing this constellation through binoculars is nothing short of spectacular, where you will witness countless thousands of stars to rival any Star Wars movie scene.

The Kepler exoplanet finder space probe, which has discovered about 2000 planets around other stars, concentrated its efforts exclusively within Cygnus, so it is surely intriguing to know, as you look through your binoculars at the stars within the constellation, that many of them possess families of planets.

Finally in the summer night sky we cannot ignore the giant ‘W’ of the sky: Cassiopeia. Though Cassiopeia is visible in the sky all year round from Ireland, it is particularly splendid in summer in the northeast region of the sky. Identifying it for the first time is hugely satisfying, and again observing it through binoculars is nothing short of spectacular. A particular treat through binoculars is the magnificent double star cluster in the constellation of Perseus, the next door constellation to Cassiopeia. and you can find the double cluster easily using Cassiopeia as a guide – from the left most star of Cassiopeia find the 2nd and 3rd stars of the great ‘W’ – then extend their line downwards for about twice their separation and you arrive at the fabulous double cluster in Perseus. Each of the clusters comprises more than 300 blue-white giant stars and are perhaps only 12 to 13 million years old – a blink of the eye in cosmological terms and when compared to our Sun’s 4,500 million year age!

I urge each and every one of you to come to know the motion of the Sun, Moon, Planets and stars in our sky. Read about it, learn how people of old figured out such motions. Convince yourself of Earth’s movement about the Sun and how it affects the seasons, length of day, how high the sun can reach in the sky and what constellations you can see at each time of the year. There is no doubt that gaining such insight delivers a great sense of connection and ‘place’ in the greater natural scheme.

Likewise, I strongly advise you to come to know the night sky at various times of the year. There is nothing more enthralling than identifying the constellations for yourself. The first time you see a constellation pattern in the sky is a very exciting moment; and without trying to be over the top about it, a quite personal moment. For me, that moment occurred when I was about 10 or 11. I had read about the constellation of Orion from a beautiful little book called the “Observers Book of Astronomy” my Patrick Moore, and set out on winter evenings to see it for myself. I spent no less than three winters looking for it, but to no avail. I just could not find it. For the first couple of winters I looked and looked. What kept drawing my attention, however, was three stars in a straight line that would appear in the autumn sky to the southeast , and fade in spring to the southwest. I found those stars fascinating, but for love nor money I could not find Orion. I read about it more and more, but nothing could reveal it to me.

And then, on the third winter, while looking at those intriguing three star, Orion presented itself to me in a moment of pure revelation. I looked at the three stars and realised in a moment of joy and exhilaration that they were actually at the centre of Orion! There was Orion, magnificent – HUGE – surrounding those three stars (the belt of Orion!) in the sky, dominating a much, much larger part of the southern sky than I had expected – in front of me all the time. My mistake over the previous years had been a lacking of perception of the size of the constellations in the sky. I had thought they were tiny, and that I would have to track them down with fine precision within a small part the sky! Nothing could have prepared me for the sheer size and scale of the the constellations in the sky and how readily visible they are to the unaided eye; and so I could not see Orion even on the most glorious of dark, crisp nights. I will never forget that night and that moment of personal discovery; and I urge each and every one of you to similarly explore the sky – most especially with your naked eye and at most with a pair of binoculars – but first and foremost with your eyes only (and a good night sky guide), to discover the constellations for yourself.

And be sure to look into the summer night sky on some calm cloudless evening – either with the naked eye to discover the constellations for yourself or with a pair of binoculars to witness the splendor of the heavens just beyond the perception of the naked eye. Find out a little of the mythology of each constellation you identify, learn what stars, nebulae and star clusters are in it, read a little of the astrophysics that describes the processes underlying the formation, evolution and fate of the stars and indeed the Milky Way itself. Finally, perhaps explore the magnificent and unique Hubble Space Telescope image archive on-line to witness visual details not visible from the ground and indeed to learn more about the powerful cosmological processes that drive the workings of the Universe itself. Make such a quest your own. From looking into the night sky to learning about the underlying cosmological forces at play, there is much you can do to enhance your sense and understanding of nature on the grandest scheme. You do not need to be a scientist or astronomer to do this, and each and every one of us has an entitlement to such a sense of connection and ownership of the universe within which we live.

Happy observing and learning!

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