In these days of electronic gadgets, multiple apps and go-to telescopes it is easy to forget that the best observational devices we have are our eyes. They have a large field of view, can change direction almost instantaneously but can focus in on detail as well. They don’t involve additional cost, need little preparation and no tidying away after you have finished your observations! But do treat them well. Give yourself fifteen minutes to get accustomed to the dark and avoid the use of bright white lights outside. Red LED lights are readily available if you need them to avoid obstacles or read documents and this will enable you to maintain your night vision. Having said that many people find a pair of binoculars very useful for picking out fainter stars when light conditions aren’t optimum.
The objective of this series of articles is to help people find their way around the night sky using only their eyes so there will not be lots of detail on individual stars or planets as that can be found elsewhere. If a star has a number after it that tells its rank in the order of brightness of stars in the northern hemisphere. The idea is that constellations and asterisms (star patterns) are like addresses and signposts for where you want to go. There are 88 constellations in all so only the main ones which are easily recognised will be looked at. As well as this it is hoped that it will be possible to highlight any unusual phenomena which may occur as the year progresses such as planets in good observational positions, planetary conjunctions, special Moon effects, meteor showers or comets.
The Celestial Sphere
Before we venture outside let us recall some helpful facts. It is useful to think of the sky as a hollow sphere which has the Earth at its centre and to which all the heavenly objects are attached. This sphere is known as the celestial sphere. Just like when you visit a planetarium. Far away objects such as stars and galaxies are in more or less ‘fixed positions’ on the celestial sphere whereas the Sun, Moon and planets continually shift their positions but stay close to a circular path on the sphere’s surface called the ‘ecliptic’ which is tilted to the celestial equator because the Earth’s axis is tilted by 23.5 degrees to the plane of its orbit. In reality of course the Earth revolves round the Sun and the ecliptic is where the plane of the Earth’s orbit cuts the celestial sphere. This makes sense because when we observe the Sun we are looking along the radius of the Earth’s orbit and hence in the plane of its orbit. The celestial sphere also has north and south poles directly above the corresponding poles on Earth and a celestial equator directly above the Earth’s equator.
For us in the northern hemisphere we see the stars rotate about the north celestial pole. Don’t worry about some of the additional information on the diagram. The yellow line is the ecliptic and it shows the signs of the zodiac (representing the constellations) and how the Sun appears to pass in front of them as the Earth revolves around the Sun. Remember we are using a model for what we see and this is governed by the movement of the Earth. The Earth spins about its axis from West to East once a day (ie 360 degrees in 24 hours or 15 degrees per hour) and that is why we see the Sun move across the sky daily from East to West. It may not be so obvious that the stars are doing the same thing at night and they move across the sky from East to West at 15 degrees per hour as well. Of course, they also do it during the day, but we cannot see them for the glare of the Sun.
The Earth also revolves about the Sun once a year (ie 360 degrees in 365 days or about 1 degree per day or 15 degrees in 15 days) which is why the sky at 10.00pm one day will look like the sky at 9.00pm 15 days later. If you wait till 10.00pm again the celestial sphere has moved on by 15 degrees or 1 hour and all the stars have moved that amount further west.
Okay, it is time to look at the stars. We have to start with the planet Venus because it has given a brilliant display over the past two months and continues to do so. Venus is breaking records at present because it reached its brightest on the 28th April and is nearest to the highest altitude in the sky at sunset that it can be. It is well worth observing in twilight when there is nothing else in the sky. Just go outside and look westwards and provided the sky is clear I defy anyone not to see it! After a cup of coffee you can go back out when the sky has darkened. Those of you with telescopes may want to observe Venus as a crescent before it enters inferior conjunction (passes directly between the Earth and the Sun). Make the most of it because by the end of May it will have ceased to be an evening star as it passes between the Earth and the Sun but it won’t be gone for long, reappearing as a morning star by mid-June.
While facing Venus turn southwards and trace a path back along the ecliptic, the path that the Sun took earlier in the evening, until you see two bright stars. You will have to go about 35 degrees ( see Lilli’s article on measuring Angular Size). These two stars are Castor (17) and Pollux (12) the dominant stars in the constellation Gemini- the Twins. The other stars are much fainter and may not be visible if lighting conditions are poor.
Now continue backwards along the ecliptic about another 35 degrees and you will find another bright star, Regulus (15), the brightest star in the constellation Leo- the Lion, which unlike many constellations does look like what it represents, a crouching lion. Above Regulus and representing the lion’s mane is theasterism known as the Sickle, looking like a backwords question-mark with Regulus the dot at the bottom.
For the final star hop all you have to do is raise your head till it is looking upwards at the zenith, the point on the celestial sphere directly above your head. You will immediately recognise the Plough, not a constellation this time but probably the best known asterism. ‘Pan’ would be a better name in modern times and in the USA it is known as the Big Dipper.
The Plough is part of the constellation Ursa Major- the Great Bear. But like many constellations it takes a lot of imagination to see a bear. In the diagram there are two stars named on the Plough, Merak and Dubhe, and these are called the pointers. A line from Merak to Dubhe continued onwards leads to Polaris- the Pole Star. The distance is about x5 the distance between Merak and Dubhe. Polaris is very close to the celestial north pole and easily found because although not very bright it is the only star visible in that area.
Now let us retrace our steps. Follow a line from Dubhe through Merak downwards and you come back to Leo. Now go westwards along the ecliptic and you come to Gemini again. Then how could anyone resist taking another look at Venus! Hopefully this hopping about the sky from one known star group to another will give you confidence to continue on the journey to other constellations in the weeks ahead. The next article will be on the website ready for the beginning of June and we’ll be starting from the Plough so you will know where you are. The same pattern will be adopted for the following months.
Something to look out for
There is a chance to see Mercury just after sunset at the end of May. Its orbit lies between the Earth’s orbit and the Sun and it will be at half phase (dichotomy) on the 29th May, so shining brightly. However it will be tricky to see as it is close to the horizon with an altitude of 16 degrees at sunset. Sunset is at 9-09 pm BST and Mercury sets at 11-17 pm BST but it is losing altitude all the time after sunset. Mercury will be located in the west to the bottom right hand side of Gemini. There is a chance to identify it more easily between the 22nd and the 24th of the month when it will be close to Venus but not so bright and both near the horizon.