WHY THE STARS MOVE
The movement of Earth makes the stars appear to march across the sky from East to West.
The Earth turns on its axis once a day. And takes a year to go round the Sun. A day in this context is the solar day, the time it takes our planet to complete one rotation on its axis relative to the Sun, which lasts for 23.93 hours. A year is the time it takes for Earth to complete an orbit of the Sun 365.26 days.
It is the fact that Earth is spinning on its axis that gives us the impression that the Sun and every other celestial object move across the sky.
The distance between Earth and the Sun does change, as our planet’s orbital path is slightly elliptical (like a squashed oval) rather than circular, which leads to a difference of 5 million km between Earth’s closest point to the Sun (perihelion), and its farthest (aphelion).
On the day that the north pole is tilted 23.5° towards the Sun, the south pole points away by the same inclination.
For the northern hemisphere, the day this happens is the longest in terms of daylight hours (the summer solstice) and for the southern hemisphere it is the shortest (the winter solstice).
As Earth goes round the Sun, its axis always tilts in the same direction in relation to the stars.
Earth’s rotation with respect to the stars only takes 23 hours and 56 minutes for the stars to return to the same position that they were the night before, a period known as the sidereal day
The reason for this discrepancy is that, from one day to the next, Earth completes 1/365th of its orbit around the Sun.
So each night, if you were to look due east, you would be looking out onto a slightly different region of space. This time difference between the solar and sidereal days, although short, causes the stars to rise almost four minutes earlier each day.
Over the weeks and months, this causes the constellations visible in the night sky to change. After 12 months, the stars will have cycled all the way back to the same positions they were in a year ago.
The path of the Sun, where you’ll find the rest of the Solar System’s planets, is the second of two important lines that astronomers use to divide up the night sky.
The ecliptic is the invisible path that the Sun traces as it moves around the sky. Think of it like this: if the Sun were to drop breadcrumbs behind it like a cosmic Hansel and Gretel, this is the trail it would leave behind.
All of the planets in the Solar System occupy orbital planes similar to our own. This is because when the Solar System formed, billions of years ago, dust and gas surrounding our nascent star was pulled into a disc under the influence of gravity.
The planets we know today all formed within this disc, and hence they all occupy planes similar to the ecliptic.
It’s this ‘coplanar’ nature of the Sun and planets that allows many of the events that captivate astronomers to occur so often. When our Moon and the Sun line up, we see an eclipse.
When a planet appears to be in the same region of sky as another, or our own Moon, we call it a conjunction. Even seemingly rare events, such as a transit of Venus, are really quite frequent in cosmological terms.
The two points at which the ecliptic crosses the celestial equator mark the moments when the hours of day and night are roughly the same.
These are known as equinoxes, from the Latin for ‘equal night’. In the northern hemisphere, the equinox in mid-March heralds spring, while the one in mid-September signals the beginning of autumn. At these two points in its orbit, Earth has no tilt relative to the Sun.
From the March equinox, the days slowly lengthen until mid-June, when Earth reaches the point in its orbit where it is at its greatest tilt relative to the Sun a solstice.
This is both the first day of summer and the longest day of the year. At this point, the ecliptic and the celestial equator are at their farthest apart. There’s another solstice six months later in mid December, when the tilt of the poles is completely reversed in relation to the Sun.
YOUR FIRST NIGHT OUTSIDE
Standing under a starry sky, awash with pinpricks of light, can as bewildering as it is mesmerising. So, once you have a clear night, where do you begin? Assuming you live in the northern hemisphere at a mid-to-high latitude – which do if you live in the UK your first goal is to find the group of seven stars known as the Plough.
The Plough is an asterism within the constellation of Ursa Major, the Great Bear; an asterism being a bright and recognisable pattern of stars often (but not always) from a single constellation.
It’s worth noting that each of the Plough’s seven stars has a name; not all stars do.
Now that you know where Dubhe and Merak are, you’ve discovered two of the most useful stars in the night sky. These two stars are known as the Pointers, because they can make it easy to locate the Pole Star, which astronomers know by the name Polaris. We’ll do this using a technique that has been tried and tested over thousands of years, known as star hopping.
Starting at Merak, draw an imaginary line through Dubhe and keep going. The next star of any note you come across is Polaris.
You’ve already seen how to locate Polaris. Now continue this imaginary line onwards for the same distance that you’ve already come from the Plough, take a slight bend to the right, and you arrive at the constellation of Cassiopeia (the Queen), which appears in the form of a W of stars.
To get to Castor and nearby Pollux, the main stars of Gemini (the Twins) start from the Plough star Megrez. Draw an imaginary line to Merak, diagonally opposite it, and keep going. Almost halfway to your target you’ll pass the two stars that form the front paws of Ursa Major.
STAR HOPPING FROM ORION’S BELT
Extend a line through Orion’s Belt northwest for 22°, where you will find the bright orange star Aldebaran at one tip of a V of stars. This is the Hyades open cluster.
Now extend it 14° farther on and you will find the Pleiades open cluster, commonly called the Seven Sisters.
From Orion’s Belt, look about 20° southeast to reach the bright star Sirius which, with Betelgeuse, is part of the Winter Triangle asterism.
Imagine that Sirius and Betelgeuse are the base of an equilateral triangle. At the other apex is the third star, Procyon.
To get to Leo (the Lion) you also start from Megrez, but this time trace a line through Phecda, the star below it in the Plough. Continuing on this line will take you to Regulus, the brightest star in Leo.
The head of the Lion is made by an easily seen hook-shaped asterism called the Sickle that works up from Regulus.
To find Auriga (the Charioteer) start again from Megrez, but this time take a route through Dubhe, to its right. After an expanse of emptiness that includes the very faint constellation of Camelopardalis (the Giraffe) you will eventually arrive at the yellow star Capella, the brightest star of Auriga.
UNDERSTANDING THE NIGHT SKY
LET YOUR EYES ADJUST
This is crucial. If you go outside from a brightly lit room, you’ll probably only see a handful of stars. Wait and let your eyes adjust to the darkness – ideally for 30 minutes – and you’ll notice an incredible difference.
PICK UP A STAR CHART
They are a great way to learn your way around the night sky. You can begin by identifying patterns of bright stars.
From there you can gradually learn your way around the constellations, and before too long they’ll become familiar and you’ll be able navigate your way around the night sky without reference to a book or chart. They frequently list the locations of prominent deep-sky objects, which, being dim, can be harder to locate.
TAKE A RED TORCH AND A COMPASS
Your eyes are dark-adapted, yet you’d still like to .see charts and be sure that you’re not about to step on a hedgehog. The answer is a red-light torch, as dark-adapted eyes are much less sensitive to red light than they are to white. You can buy dedicated red-light torches, or make a DIY one by taking a normal torch and fixing a piece of red acetate over the front. A compass will help you find north, and is useful in using star charts.
AVOID ARTIFICIAL LIGHTS
Make sure any light sources are obscured from your observing position, as they will prevent your eyes from acclimatising to the darkness properly.
If you can get out to the countryside you can take advantage of properly dark skies this will really make a difference.
TAKE YOUR TIME
The fact is that there is an awful lot to get your head around, and no one has ever looked at the night sky and instantly understood how to find their way around.
Not even Sir Patrick Moore was immune to this; he did it by learning one new constellation each night.
HOW TO DEAL WITH LIGHT POLLUTION
This vexation comes in two flavours: sky glow, the rusty orange haze cast by the massed lights over a wide area, and local glare from line-of-sight sources nearby streetlights, security lights, car headlights, even the light emanating from your neighbours’ windows.
Sky glow washes out the night and blots out the stars, while local sources are more prone to ruining your night vision.
For local sources of light pollution, your biggest consideration is where you position your self in your garden. You need to find a spot that puts a barrier between yourself and the irksome source of glare.
That barrier could be anything a fence, a tree, the side of a building – so long as it isn’t so big it also masks the part of the sky you want to look at.
In many places there is a noticeable drop off in sky glow after midnight as more and more people and businesses turn off their interior lights, meaning the wee hours often offer better views. You may also find that your local authority turns off streetlights at a set time.If sky glow is a particular problem, make sure you wait until your chosen target is well clear of the horizon before you attempt to view it.
1 GET YOUR BEARINGS
There’s one thing you need to know before using a planisphere, the cardinal points from where you live.
If you don’t have a compass, use the Sun. It rises roughly in the east and sets roughly in the west.
2 SET THE PLANISPHERE
Let’s say you’re heading out at 9pm on 15 January. Align the 9pm marker on the upper disc with the 15 January marker on the lower disc. The stars in the oval window should now match those in the skies above.
3 HOLD IT UP
To start with, look north, holding the planisphere so that the word ‘north’ is at the bottom. If you change the direction you’re facing, move the planisphere round so that the corresponding compass point is now at the bottom.
4 STAR HOPPING
The central pin represents Polaris andthenorth celestial pole. Just to its lower right will be the seven bright stars of the Plough. Use these and the five stars forming the W shape of Cassiopeia to get to know theconstellations.
(the Pole Star)
You may be forgiven for thinking that full Moon is the best time to examine our close companion not so. While this is a good time to see the long, bright rays of ejecta surrounding prominent craters such as Tycho, the high altitude of the Sun in the lunar sky means no shadows are cast, resulting in a washed-out view of the Moon.In general, the best time to view a given lunar feature is when the terminator, the demarcating line that separates lunar day and night, is nearby.
Because Mercury and Venus are closer to the Sun than Earth, they are known as the inferior planets. The best time to observe them is when they are at their farthest angular distance from the Sun. At these times, the planets are only half lit by the Sun, but after this they swing back into the solar glare, where they become less visible.
The planets further out from Earth are called superior planets. The best time to observe the superior planets is when they are close to Earth.This happens at opposition, when the planet is on the opposite side of the sky to the Sun, so we are presented with a fully illuminated disc: visually it’s close to or at its biggest and brightest.
A piece of rocky debris in space that is smaller than an asteroid.
A small piece of space debris, typically the size of a grain of sand, that has entered Earth’s atmosphere. Heating causes it to glow, causing streaks to appear in the sky. They’re popularly known as ‘shooting stars’.
A meteor that survives being burnt up in Earth’s atmosphere and crashes into the ground. Such fragments are useful sources of information about the history of the Solar System.
The radiant is the point in the sky where meteors (associated with a specific meteor shower) appear to come from. The constellation where the radiant is located determines the name of the meteor shower. So for example, the Orionids have their radiant in Orion.
Meteor showers have what’s known as a ‘peak’, the night when you can expect to see the greatest number of meteors.
The rates can vary quite substantially, but prominent displays such as the Perseids can produce an average of one meteor a minute under clear, moonless skies at their peak.
Don’t look directly at the radiant, but concentrate your gaze high in the direction of the darkest portion of the sky that’s free from obscuring trees and buildings.
Wanderers of the Solar System, comets can be amongst the most spectacular of astronomical sights when they appear in our skies.
These mysterious visitors never fail to capture imaginations when they pass by, and after years of careful observations astronomers have coaxed out the secrets hidden by their glow. The heart of a comet is its nucleus, a core of ice laced with rock and dust.
Though sometimes called a ‘dirty snowball’, the ice found on comets is far more exotic than that on Earth. These snowballs travel in huge elliptical orbits, briefly visiting the inner Solar System at one end before travelling billions of kilometres to the outer regions. As the comet gets closer to the Sun, it begins to feel the solar influence even more acutely, as its wind and magnetic field sweep the dust and gas out into a huge tail.
This can extend for millions of kilometres, spanning huge swathes of the Solar System. Some of the tail’s debris is left behind in its orbit to form a meteoroid stream.
Several of these cross the Earth’s orbit, and when we pass through them every year, we see the debris burning up in the atmosphere as a meteor shower.