I am quite surprised that this month’s blog will complete a year of them, meaning that we have been all the way round the night sky. By the nature of the subject there will be a bit of repetition from now on but there is always something new happening and who can ever tire of looking at a lovely starry sky even if you have seen it before. We passed the spring equinox on the 20th March so we are getting more daylight and the clocks went forward an hour on the 28th March so it will be later before the sky darkens. The good news is that we have had some clear nights and seeing Mars, Aldebaran, the Pleiades and a crescent Moon all together just after the middle of the month was particularly pleasing. Since we are making a fresh start I thought that it would be a good idea to repeat the bit about the celestial sphere and how the sky changes in appearance from night to night and month to month. I hope this will prove useful to any newcomers to the subject and any youngsters who are hopefully embarking on observing the skies as a lifetime’s hobby.
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. 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. 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 recent equinox marks the point where the path round the ecliptic crosses the celestial equator. This is when the Sun is overhead at the equator and it continues to travel further north until the summer solstice when it is overhead at the Tropic of Cancer. We see from the diagram that the ecliptic is north of the celestial equator during this period of time.
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. The following charts represent the night sky at 10.00pm BST on the 8th of April and at 9.00pm BST on the 23rd April. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart. From ancient times the stars have been put into groups called constellations with names supposedly indicating what they represent but this is seldom clear.
The fact that some stars appear in a group does not indicate that they are close together and their distances can vary by very large amounts. Some groups of stars stand out but may be only part of a constellation and such groupings are called ‘asterisms’.
So facing south and going up from the horizon you will see the constellation Leo- The Lion. Fortunately it does look like a crouching lion facing towards the right with the brightest star Regulus (the 15th brightest seen from the northern hemisphere) being its front paw and the curve of stars above that representing its head and mane. This latter grouping of stars is an example of an asterism called ‘the Sickle’, looking like a backwards question mark with Regulus being the dot at the bottom.
Now raise your eyes upward to your zenith (the point directly above where you are standing) and you will see what must be the best known asterism in the night sky- The Plough. It contains seven stars and the chart shows three of them named. The Plough is part of the constellation – Ursa Major- The Great Bear, but it takes a lot of imagination to see a bear and that region is mostly referred to as The Plough. In North America it is called the Big Dipper and perhaps here in the UK a better name in modern times would be ‘The Pan’. We said in the introduction that the stars rotate about the celestial North Pole and stars close to there never set but are visible all year round when the skies are dark. Stars like this are said to be circumpolar and Ursa Major is a circumpolar constellation. But note The Plough’s orientation carefully because as it continues on its circular journey it will appear upside down in six months’ time.
The constellations are used as signposts in the sky and enable us to engage in a fun activity called ‘star hopping’. Now let’s look at the second chart.
The two stars in the Plough, Merak and Dubhe, are called the pointers and 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. Polaris is in the constellation- Ursa Minor- The Little Bear. Now consider a line from the star Alioth in the Plough, through Polaris and continued onwards for about the same distance again until you see a bright star. It will be the central star of a W formation, an asterism in the constellation Cassiopeia- Queen Cassiopeia in Greek mythology. Most people see the W shape and call it Cassiopeia. The bright star was never given a name in Western or Middle Eastern culture so is referred to as gamma (g) Cas. The convention is to name stars using the letters of the Greek alphabet and an abbreviated form of the constellation. Generally this is done in the order of brightness of the star but it is not a hard and fast rule.However this star has been given the name Navi, allegedly by the American astronaut Virgil (Gus) Ivan Grissom as an anagram of his middle name because it was used for navigation in the early space missions. A fitting tribute to someone who made the ultimate sacrifice for space exploration. The constellation Cassiopeia is also circumpolar and because it is directly opposite the Plough across the North Celestial Pole the two will have exchanged positions in six months so we will see Cassiopeia much better in November. Just imagine the two of them at the ends of a long pole rotating about the North Pole.
Something to look out for
There will be a close approach of a five day old Moon and Mars on Saturday 17th April. They will be separated by about 4 degrees initially but will come within a quarter of a degree of each other at their closest. The pair will be visible after 8.30pm as dusk fades above your western horizon.
At the end of the month we welcome back Venus to the evening sky and though it is still close to the Sun it will be visible for a short time after sunset above the western horizon. It will have a close approach with Mercury on Sunday the 25th April but you will have to let the dusk sky fade before they become visible. Clear skies.