Amateur Astronomers Association of Princeton

by S. Prasad Ganti

Some stars, constellations and galaxies are visible from the northern hemisphere which are not visible or barely visible from the southern hemisphere. Likewise, some stars, constellations and galaxies are unique to the skies as seen from the southern hemisphere. We are so used to seeing the north star with the big dipper and the cassiopeia constellations going in circles around it. North star lies directly above the north pole of the earth’s axis of rotation  and hence is not visible from the southern hemisphere. It stays in the same position all the nights of the year.

A significant constellation closer to, but not exactly above the south pole, is the southern cross. This is not visible from the northern hemisphere. Though I read that it is possible to see it very low in the sky as we go closer to the equator. I tried to look for it when I visited Key West in Florida, but was unsuccessful. I added to my bucket list to see the southern cross. 

Recently I had an opportunity to visit Brisbane, Australia to attend a family wedding. Although I did cross the equator in the early 1990s to visit Sao Paulo, Brazil on a business trip, I hardly knew about the southern cross. Also, the light pollution of Sao Paulo would not have helped anyway.  I prepared for the Australian trip by reading about the southern cross and the pointer stars. Hoping that this would help me in identifying using my naked eyes. I also loaded the Stellarium software on my phone. The southern cross is relatively easy to spot from anywhere in Australia. In fact, it is prominently displayed on the Australian flag. In the picture shown below, four stars forming some sort of a diamond shape at the right is the southern cross (with a fifth star inside), while the union jack is on the top left part of the flag.

On my first night in the suburbs of Brisbane, I was able to spot the southern cross. It was not bright due to the light pollution. I could not photograph it either. I just carried back the memories in my mind. I could see the southern cross on the second night as well, and from different locations in the neighborhood, but the third night was cloudy. I thanked my stars for the opportunity provided to me on the first two nights!

Below is the picture from the ABC science site https://www.abc.net.au/science/starhunt/tour/virtual/southern-cross/

The alignment of the cross changes with the season. At this time of the year, it is horizontal with the 2 bright pointer stars on the top. The bright white star on the left (actually on top in the real sky in August) is the Alpha Centauri, our nearest star about 4 light years away. The bluish star to its right is the Beta Centauri, which is also part of the constellation Centaurus.  To the right is the southern cross, also known as the constellation of crux. The brightest star is on the bottom and is labeled as Alpha Crux.

In Hindu weddings, one of the ritualistic steps is for the priest to take the bride and groom outside and point to the binary stars Alcor and Mizar (known as “Arundathi and Vashistha” in Indian mythology). Weddings can take place during day times also, but the priests know where these stars are supposed to be  in the sky. These stars are in the big dipper which is visible from the northern skies. But I was not sure if one can see it in the southern skies. I asked the priest, a scholarly gentleman, if these stars actually show up. He was sure he saw them. 

The picture of the big dipper is shown in the picture below, courtesy unsplash.com. It looks like a question mark. The second star from the bottom (in the tail of the question mark or handle of the vessel) is the Mizar/Alcor binary star system. Using a telescope, it is possible to clearly see the 2 stars instead of 1 with naked eye. 

It was a great experience down under seeing the southern cross and learning the Hindu names of Alcor and Mizar.