The Sky Tonight is a monthly update of the amazing things you can find when looking up from here in Western Australia. This section is written by Jacquie Milner, resident astronomer at the Scitech Planetarium.
The casual observer
Astrofest is on again in 2017 and will be held on Saturday 18th March at the Curtin Stadium at Curtin University. There will be talks, stalls to browse, a photo competition to admire and telescopes to look through outside and if you pre-register you can be in the running for some great prizes. And it’s all free! Details at astronomywa.net.au/astrofest
The autumnal equinox for the southern hemisphere occurs on Monday 20th March in 2017. After this day the nights become longer than the days.
Dates of interest
1st – Moon to the left of Mars, evening sky.
5th – Moon to the right of Aldebaran, the brightest star in Taurus the Bull.
14th – Moon to the left of Jupiter, evening sky.
20th – Equinox, the Sun passes from the southern to the northern hemisphere.
20th – Moon below Saturn, evening sky.
29th – Moon to the upper-left of Mercury, very low in the evening sky just after sunset.
30th – Moon to the left of Mars, low in the evening sky.
Planets to look for
The evening sky is busy this March, as the planets bunch up on one side of the sky again. You will only have a few days to catch Venus before it slides into the Sun’s glare as it passes between Earth and the Sun, a line-up known as inferior conjunction. Mercury also stays very close to the Sun in a very bad alignment for this time of year, but you may catch a glimpse of it near the Moon on the evening of the 29th.
Mars is saying its farewells, too, as it prepares to go around the far side of the Sun from us. Now is the time to be patient with the Red Planet, and remind yourself that it might look a very disappointing small dot in the telescope right now, but in a year’s time it look its best since 2003!
Jupiter is now on show in the evening, rising just after sunset with the constellation Virgo and the bright star Spica. On March 14th the Moon will be to the left of Jupiter, then white Spica on the right.
Saturn is three hours behind Jupiter, near the Teapot asterism of Sagittarius. The last quarter Moon will be underneath it on the night of the 20th, rising not long before midnight. By the end of the month it is rising by 10pm.
The dawn horizon is mostly empty of planets this month. A waning Moon will sink into the twilight glow by itself late in the month, while Jupiter is low in the west and Saturn is high above by sunrise. Right at the end of March Venus reappears from inferior conjunction, but it will be much easier to see in April.
Constellation of the month
Canis Major, the Great Dog
Canis Major, the Big Dog of the skies, passes directly overhead in southern Australia. He is depicted looking towards his master, Orion the Hunter, and the fleeing shape of Lepus the Hare, underneath Orion's feet. As he rises in the east with Orion he seems to be lying on his back in the Milky Way, with several bright stars marking his spine and legs.
On his shoulder shines Sirius, the brightest star in the sky. Most of the mythology of Canis Major is connected to Sirius as the constellation itself did not take on its current form until Roman times. Although it is the brightest star Sirius is not the closest star we can see in the sky, but it is still close to us, lying around 8.7 light years away. It is a hot, white star that will shine brightly for a long time to come.
Sirius means "Scorching" and was named so by the ancient Greeks as it rose and set with the Sun during the northern hemisphere summer 3000 years ago. It was thought that the combined heat of the two stars caused the hot weather of late summer. It was also commonly known as the Dog Star, a tradition that reaches back far before Babylonian times. This is where the expression "the dog days of summer" comes from.
Sirius was an important star to the ancient Egyptians. They watched for the heliacal rising of Sirius to indicate the start of the annual flooding of the Nile River (a heliacal rising is when a star is first seen in the eastern sky before dawn after it has passed behind the Sun).
Canis Major is high overhead as it gets dark now, but still following Orion down to the west as the night goes on.
Objects for the small telescope
Sirius B, or the Pup
Sirius is actually a double star; the bright white star we see blazing in the sky above has a smaller white dwarf companion that completes an orbit around it every 50 years. The problem is two-fold for us earthly observers – the companion star, Sirius B, or “the Pup,” as it also known, is much fainter than its dazzling primary. And its orbital plane is tilted a little from our point of view, so that it spends some years too close to Sirius A to be seen.
From now until around 2025 Sirius B will at its most distant from Sirius A, at around 11 arcseconds separation. This might not sound like much but it will give you a much better chance of separating the faint 8.4 magnitude white dwarf from the swamping glow of the -1.46 primary than when it is at its closest, which is only 3 arcseconds.
You can see from the chart below Sirius B will be closest to Sirius A again in 2043, and the next opportunity to get a good look at it will return around 2070. Alvan Clark was lucky to find it by accident while testing a new telescope he had made on January 31st 1862, when it would have not quite been at maximum separation, although another astronomer, Freidrich Bessel, had suspected that it might be there back in 1844 from changes he noticed in the proper motion of this close neighbour to us.
The orbit of Sirius B around Sirius A as seen from Earth.
(Nord=North, Sud=South, Ost=East, Jahre=Year, Scale is in arcseconds)
Credit: Wikimedia Commons
How to Find South Using the Southern Cross
The northern hemisphere is lucky at the moment, they have a moderately bright star very close to the north celestial pole to help point the way north. On the opposite side of the sky, here in the south, we just have a large, empty patch of sky. It won’t always be that way – in about 7000 years or so the effects of precession will swing the pole over near Carina, where there are lots of bright stars, but for now, we need another way to work out south.
In southern Australia the Southern Cross is nearly always visible, and it makes a useful guide to the rough location of the south celestial pole (Figure 1). This is the point in the sky that all the stars seem to rotate around during the night, and it sits directly above true south (not magnetic south!) So if you can locate this area in the sky, you will know where south is on the horizon. Best of all, it doesn’t matter where the Southern Cross is in the sky, it still works!
Figure 1: The south celestial pole sits directly above true south.
Find the Southern Cross in the sky – at this time of year is rising on the east side of the pole in the early evening. Then take the long axis of the Cross, as if it is standing upright on the Australian flag, and extend it out the bottom of the Cross four and a half times its length (See Figure 2 below). Where it stops is roughly where the south celestial pole is, and from there you can drop down to the horizon to mark south.
Figure 2: Extend the long axis of the Southern Cross out four and a half times to find the approximate location of the south celestial pole, then drop a line down to the horizon to find south.
Figure 3: It doesn’t matter what side of the sky the Southern Cross is on, it still works!
Figure 4: Even when the Southern Cross is upside down in the sky it can still help you find south.
There are a number of different methods other than this basic one, and they all work since they all find the south celestial pole one way or another.
A second popular method that is often described is to include the Pointers in your line drawing in the sky. The Pointers are the two stars also known as alpha and beta Centauri, and they “point” the way to Southern Cross. If you draw a line between the Pointers, then draw a bisecting line at right angles to that, all the way out to where it crosses the line coming from the Southern Cross, you should also have the south celestial pole (Figure 5 below).
Figure 5: Using the perpendicular bisecting line from the Pointers method.
Do remember that the Southern Cross will change position during the night, and will also gradually change position in the evening sky as the year progresses. It is at its lowest at sunset during summer, so as the weather cools down into autumn and then winter, it will be high up and easy to find – a great time to get and practice your direction finding skills.