Sky Diary

Sky at Night Chart

The Sky at Night

The Summer Triangle formed by the stars Vega, Deneb and Altair hold the south-western sky and are the first stars to be seen in the gradually darkening July skies.  The brightest of these stars, Vega, is the primary star of the constellation Lyra, which represents the lyre of Orpheus which he played so beautifully that it was placed in the heavens by the muses.  Vega, the second brightest star in the northern hemisphere makes Lyra easy to find. The star is bright because it is only 25 light years from our Sun but it is also twice as massive as the Sun and has a much hotter surface at around 10,000°C.  Because the Earth’s axis is inclined and with the gravitational attraction of the Sun and Moon, the Earth wobbles gradually and the point towards which the North Pole points in the sky gradually changes.  At present that point is marked by Polaris, the Pole Star, but Vega was the Pole Star around 12,000 BC and will be again around the year 13,700.  A short distance to the north-east of Vega is a fainter star which when viewed with binoculars can be seen to be a double star with components of almost equal brightness.  These stars are actually binary stars which means that they are gravitationally linked and orbit each other but each orbit takes around 400,000 years.  With a moderate sized telescope, each of the two stars of Epsilon Lyra are seen to be close doubles themselves and so the popular name for this star system is the ‘Double Double’.

To the south of Vega are two fairly close stars. Beta and Gamma Lyrae and on a line joining these stars can be found the splendid planetary nebula Messier 57, known as the Ring Nebula.  It requires a reasonably sized telescope to see this gem but it is well worth searching for as it appears as a tiny, but quite sharply defined, ring.  The ring is the expanding remains of a Sun like star which, within the last two thousand years, exhausted its supply of hydrogen and ejected its outer layers before shrinking to become a compact, hot white dwarf star. It’s remarkable to think that this delicate, tiny ring is 2,300 light years distant and is expanding at a rate of 30km per second. A similar fate awaits our own Sun in around four billion years’ time.

Last month I mentioned the great globular cluster Messier 13, in Hercules. Towards the eastern part of Lyra there is another of these distant clusters, this one, discovered by Charles Messier on the 19th January 1779 is numbered 56 in his catalogue.  Messier 56 is 32,900 light years away from Earth and is more than 13 billion years old.  It is easily seen with a small telescope, or even binoculars, but requires a telescope of 8 inches diameter to resolve all the stars to the cluster’s centre.

The Planets

  • Mercurymoves eastwards from the Sun throughout July but only sets an hour after the Sun so will not be visible.
  • Venus will be bright in the east rising two hours before the Sun during July. It will be seen best between 3 and 4am.
  • Mars will be too close to the Sun to be seen during July.
  • The best of Jupiter is now past but it can still be seen low in the sky towards the west, setting at midnight, two hours after the Sun.
  • Saturn is very low towards the south at 11pm mid-month and sets at about 2.30am.
  • The Moon is at first quarter on the 1st, full on the 9th, at last quarter on the 16th, new on the 23rd and at first quarter again on the 30th July.

Summer events at Mills Observatory continue on the 8th July with Saturday Stars, a drop in family event from 1 – 3pm although Mills will be open between 12.30 and 4.30pm.  There will be a planetarium show at 2pm on the 22nd July for which booking is essential at 01382 435967.  Admission to the planetarium shows is £1 for adults and 50p for children.

Visitors arriving by car should enter from Glamis Road along the west side of Lochee Park, turning left beyond the gateway and follow signs to the Observatory where there is limited car parking space.