The position of the Southern Cross in the southern sky at 8 pm standard time during each month of the year. As on the diagram the Southern Cross is circumpolar, that is it never sets, for most places in Australia. Sketch Nick Lomb
The Southern Cross has a special place in Australia as illustrated by its appearance on the national flag and its celebration in the National Anthem by the words, “Beneath our radiant Southern Cross”. As well, it appears on the logos of numerous companies such as a major bank and on the liveries of airlines.
The smallest of the 88 constellations or star pictures named by astronomers, it is prominent in the southern sky for most of the year apart from the summer months. Even in December and January it does not sink beneath the horizon as seen from most places in Australia. To find it look for the two bright stars known as the Pointers that always point to the Cross. The two stars are Alpha Centauri and Beta Centauri, which is the one closer to the Cross in the sky. Confusedly, there are a number of other stars in the shape of a cross in the vicinity such as the ones making up the False Cross, but they are are less bright than the stars of the Southern Cross, have a less compact grouping and do not have the Pointer stars.
This prominent constellation is not only interesting to look at, but has a variety of uses. Sometime ago we discussed on this blog here how to use the Southern Cross to find direction. Here we discuss how the changing position of the Cross during each night and during the year can be used to find time and the date.
The diagram at the top shows the position of the Southern Cross at 8 pm standard time (or 9 pm summer time) for each month of the year. If you do not know the date you can go outside at 8 pm, find the orientation of the Cross and use the diagram to deduce the month. More realistically, the diagram shows where to look for the Southern Cross at a convenient time in the evening on any date during the year. As can be seen March to August is the best period to view the Cross in the evenings as it low in the sky for the remaining months, especially during summer.
The Southern Cross not only spins around the southern sky once a year, it also does a complete circle each 24 hours. Actually it makes the complete circle in 23 hours 56 minutes and 4 seconds or one sidereal day. The approximately four minutes difference between an ordinary day and a sidereal day is significant for it makes the Southern Cross rise four minutes earlier each day, which is 30 minutes a week or two hours per month. And that two hours a month leads to the monthly change in orientation illustrated at the top.
The Southern Cross at various times in June. As discussed in the text it can indicate the time during other months as well. Sketch Nick Lomb
As the Southern Cross spins around the sky once a day we can use it to find the time as long as we know the date. The diagram above illustrates how the Cross changes its orientation during a night in mid June. A similar scheme can be used during other months. For example, for August subtract four hours from the times shown on the diagram so that then the Cross is in the same orientation at 8 pm as it is at midnight in June.
Keep watching the sky.