Astrowatch

The Astrowatch given to you in this workshop was built based upon the instructions in the following articles:

The Australian Scout Handbook 1973 Edition
And
Building A Portable Sundial by Michelle B. Larson
The Physics Teacher Vol 37 Feb 1999 pages 113 & 114
TPTvol37Feb99p113 and TPTvol37Feb99p114. /

It should be noted that the instructions in Michelle Larson’s article are for the Northern Hemisphere so I have adapted them for the Southern Hemisphere. I used a latitude of 40o for calculating the hour angles.

The refinement I made to the design was to incorporate a magnetic compass into the watch’s base, set with a magnetic declination of 12o East (Melbourne’s is actually 11o 39’). This means that on the watch face 12 Noon will be aligned with geographic north. This will help to improve the watch’s accuracy in telling the time. Figure 1 shows this.

Figure 1a / Figure 1b

Melbourne’s time zone is GMT + 10 hours (ignoring daylight savings time in either Australia or the UK). It is based on Pt. Hicks in Victoria whose Longitude is approximately 150o East (149 o 16′ East). Given that the Earth completes one 360o rotation about its axis in 24 hours it follows that the Earth rotates through 15o every hour. So Melbourne is about 5o or 20 minutes behind Pt. Hicks in Solar time. This means that your watch in Melbourne will be 20 minutes ahead of your sundial.

It is also worth mentioning to your students the effect that the equation of time will have on your dial’s time. However, given the small size of the Astrowatch, and its ‘rustic’ construction it would be inappropriate to include any adjustments to the measured time due to it.

So when using your Astrowatch to tell the time, keep in mind that in Melbourne you will need to add on 20 minutes to your dial time and allow for Australian daylight savings time.

For nocturnal timekeeping I used the information in the 1973 Australian Scout Handbook without alterations.

Scans for the watch’s two faces have the following file names:

astrodaywatch.tifandastronightwatch.tif

The more care you take in the watch’s construction, the greater the accuracy with which you can use it to tell the time. I’ve taken one on camp with Year 9 students and Cub Scouts and they were always amazed when I could use it to accurately tell the time to within 20 minutes of the actual time. Don’t expect any greater accuracy than about 30 minutes using the Southern Cross though, particularly with such a small dial.

Finding South With The Southern Cross

Method #1
•Imagine a line drawn through the long arm of the Cross, and then continued through the base of the Cross for a distance equal to four times the length of the Cross. Where this line ends is the South Celestial Pole.
Method #2
•Imagine a line drawn between Achernar and the pointer nearest the Cross. Half way along this line is the South Celestial Pole. /
/ Using The Southern Cross To Tell The Time
  • Face True South and rotate your Astrowatch with the picture shown so that the date is at its highest point and its centre covers the South Celestial Pole.
•The Southern Cross itself acts as the hour hands to give us an approximate time.
•The Cross is at its highest point at midnight on April 1st, and for every month later it is at its highest point two hours earlier. (i.e. on May 1st it is at its highest point at 10 pm)
•With the current month at the top of the dial locate the position of the Southern Cross and read off the dial to estimate the time.
•With practice you should be able to estimate the time to the nearest half hour.