At the start of March the sight of Venus and Jupiter together was rather special but I was thwarted from seeing the planetary alignment at the end of the month due to too much cloud cover. Let’s hope we have clearer skies this month. I’ve set a precedent in the past years by describing, in the April blog, the celestial sphere as a help to understanding the night sky and I’ve decided to continue with that in order to help anyone who is taking up astronomy for the first time. Also the constellations being observed this month are circumpolar so it is quite appropriate. If you feel you know all this just skip to the observing section.
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 (i.e. 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 (i.e. 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.
The following chart represents the night sky at 11.00pm BST on the 8th of April and at 10.00pm BST on the 23rd April. To use the chart, face south at the appropriate time with the bottom of the chart towards the horizon and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky. Actually this month the emphasis is on circumpolar stars so you will be looking north so constellation orientations will be upside down!
Like last month the chart features the constellation Ursa Major- The Great Bear with the even better known asterism, the Plough, outlined in red. Remember the pointer stars Merak and Dubhe in the Plough lead you to Polaris the Pole star. The distance to Polaris is about five times the distance between the pointers and although only of magnitude 2 Polaris is easy to identify because there are no other bright objects in that region of the sky. As mentioned earlier the Plough is near your zenith so you need to face north to see the stars beyond that and then the Plough will appear to be upside down. The Pole star is at a point in the sky close to the north celestial pole which is where the Earth’s axis of rotation points so it occupies a more or less fixed position in the night sky and marks ‘true north’ as seen from anywhere in the northern hemisphere. In previous centuries it was an important tool for finding your latitude and maintaining your direction. As the Earth spins on its axis all the stars in the sky appear to rotate about the Pole star on a daily basis. Also stars relatively close to the Pole star never set and are visible all year round when the skies are dark and are said to be circumpolar. Polaris is in the constellation Ursa Minor- The Little Bear and it mimics the brighter and larger shape of the Plough.
A line from Alioth in the Plough’s handle through Polaris and extended the same distance again leads to another circumpolar constellation Cassiopeia- Queen Cassiopeia wife of King Cepheus in Greek mythology. It’s asterism in the shape of large ‘W’ is very conspicuous looking north but as it rotates around the north celestial pole over the coming months it will rise higher in the sky until you see its ‘W’ shape looking south. It is always reassuring at any time of the year to locate the Pole star and observe the orientations of the Plough and Cassiopeia as they rotate about it.
From Cassiopeia towards your eastern horizon in the night sky is a group of not very bright stars forming a shape roughly similar to the gable end of a house. This is the constellation Cepheus- King Cepheus in Greek mythology and husband of Cassiopeia. This constellation has given its name to a group of stars known as cepheid variables which have played an important role in establishing accurate distance measurements to other stars and galaxies.
Finally wrapping itself around Ursa Minor and the Pole star is the large constellation Draco- The Dragon, but it suffers from a lack of any bright stars.
Something to look out for
Last month we discussed some terms associated with the ‘inferior’ planets Mercury and Venus and how they are only visible close to sunrise and sunset. Venus has been lovely to observe in the evening sky but this month the focus is on Mercury. It is the smallest of the planets and closest to the sun so can be tricky to observe close to the horizon but the greater the satisfaction when you manage to do so. Also it orbits the Sun in only 88 days so it doesn’t stay long in our twilight skies before it passes through inferior conjunction and disappears from view. Mercury will be at its highest altitude and greatest elongation east on Tuesday 11th April so this gives us our best chance of observing it. Of course it has been visible weather permitting recently but close to the horizon and setting quite quickly after sunset. You will need a clear view to your western horizon just after sunset and Mercury will be at its brightest early in the month. On the 11th April it sets two hours after sunset.
On Sunday 23rd April there will be a close approach of a three days old Moon and Venus from 7.30pm BST onwards then on Wednesday 26th April there will be a close approach of the Moon and Mars visible from 9.30pm BST. Remember Venus will be at its highest altitude towards the end of April and reaches greatest elongation west on the 4th June.