Observations

Using the Southern Cross to find the date or the time

The date and the Southern Cross

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.

Time and the Southern Cross

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.

14 responses to “Using the Southern Cross to find the date or the time

  • I’m only an astronomical novice, but was wondering what date of the year (if even calculable) the Southern Cross would be highest in the sky at the start of the day from Uluru (1201 am)?

    Would it be the same every year?

    I was thinking, that if it always fell on the same day, it would perhaps be a great day to celebrate Australia Day, as the Southern Cross is especially important to both Aboriginal Australians, European Explorers/Settlers and modern day Australians. Being a celestial event, as opposed to a political one, it would perhaps be a fitting day for us all to celebrate Australia

    • Richard, According to the planetarium program I use the Southern Cross would be at its highest in the sky (i.e. on the meridian* and “upright”) as seen from Uluru at 00:01am on April 11 each year. Yes, it would be the same date each year. However, from some states (those that use Daylight Saving times) the Cross is never on the meridian and “upright” at 00:01am! This is quirk of the time of year – the switch out of daylight-saving occurs very close to this date and causes the Cross to “jump” past the meridian without ever being on it at 00:01am.
      * The meridian is the line across the sky that passes from due south, directly overhead and then through due north. Any object on this line will have reached its highest point in the sky for that day.

  • Thanx Andrew for this article, as I was observing the changes last night from my room in Narooma overlooking the beautiful Wagonga Inlet. I was really awake at 2-4 am to see the meteor shower we are passing thru, which was spectacular, even tho a bright moon was out.
    Our natural world is such an amazing wonder to observe, I feel blessed to have an inquiring mind, although sometimes the wonder of it all and the scale of time and the Universe is daunting.
    Just aside, it would seem that the name the ‘morning and evening ‘star’ Venus and Mercury are technically incorrect, because they are within our system where the Sun is the Star. hmmm much to ponder

    • Nic, You are right, Venus and Mercury are not stars. We know that now of course. But in the distant past, people noticed that a few ‘stars’ moved among the fixed stars. They called these ‘wandering stars’, or ‘wanderers’, but they were speaking Ancient Greek and their word for wanderer was ‘planetes’. So we now call them planets.

  • I am just starting to understand some of the maths of and navigation using the stars.
    A couple of questions regarding the diagrams in the article.
    Re the 8pm cycle, is this the location beginning, middle,end or average position per month?
    What adjustments need to be made when doing these calculations in areas further East or West of the east coast of Australia?

    • Dimmo, The first figure shows the location of the Cross at 8pm at the middle of each month. No adjustments need be made for other locations – just use your local time. For example, in the middle of June in Sydney at 8pm the Cross is as shown in the first figure. At the same moment in Perth the time is 6pm and the Cross will be seen at the ‘May’ position in the first figure. But by 8pm in Perth the Cross will have moved to ‘June’ position.

  • A far easier way of describing how to tell time using the Southern Cross – without having to refer to diagrams – is simply :-

    At the end of March, the Southern Cross from head to toe is the hour hand of an accurate 24 hr clock. Add or subtract two hours for every month since (or until) the end of March.

    For example tonight is mid-January and the Southern Cross head to toe points to about the 7 o’clock position on a 24 hr clock. Subtracting about 5 hours for the two and a half hours [months] ’til March 31 gives the time as 2am (in God-fearing non daylight saving Queensland time).
    My guess tonight was about 3 am, actual time 2.15am.

    Date can be ascertained by the reverse to within 2 weeks.

    Source: Graves’ “Bushcraft”

    • Josh, Thanks for this. However, I’m not sure this is any easier to understand than diagrams! And I think you need to be more clear in describing your method. For example, at what time does the Cross point to the “7 o’clock” position in mid-January?

  • Thank you! This page was very helpful, first time in the southern hemisphere and looking for the cross. Found it but the orientation was different from all the charts I have found. Had been second guessing my observation until I found all the great info on this page. I can now say with certainty that I have seen the southern cross! Not only very interesting, but quite useful as well. Most informative page I have found regarding the crux! Kudos to a job well done!

    • Thank you Graham, we are pleased to know this was helpful. Strangely, it is quite difficult to identify the Cross at first but once you get to know it’s shape and the way it rotates around the south celestial pole it is impossible to miss!

  • Hello Rodman and Bert. allowing for the fact that there was no daylight saving time in operation in 1948, the Southern Cross at that time was in the south-east at about the 8:30 am position. Bert, there is a small change in the position of the Cross from year to year on the same date due to the four-year leap year cycle, but it is only small.

  • I think this is truly amazing. and to answer rodmans question the Southern Cross would have been about 5 minutes before nine o clock. that is when looking from South Africa! Dont ask me why. Which seems like 10 minutes further on the clockface than this year. I found this on STELLARIUM.

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