The Right Ascension wheel is used here to display the right ascension of the Sun ( in hours ) at any date throughout the year. It should be noted that the dates on the wheel as presented here are only approximate, because of the inclination of the ecliptic with respect to the celestial equator, the Sun doesn't move through the same amount of right ascension throughout the year ( the effect isn't very great, the Sun moves through about 4 and a half minutes of right ascension per day during the days around the Solstices, and about 3 and a half minutes per day around the equinoxes. ) The idea here is to read off the right ascension of the Sun given the date. For instance, the wheel suggests that the Sun's right ascension of Nov 21 should be 16 hrs ( actually it is 15 hours and 50.5 minutes, but we're not going to worry about 10 minutes.)
To find the right ascension for dates not shown you need to go through a process of interpolation. For Example Jan 1 is 11 days after Dec 21. 11 days is a little less than half a month. The right ascension of the Sun will be a little less than half way between 18 hrs ( right ascension for Dec 21 ) and 20 hrs ( right ascension for Jan 21 ). A good estimate would be a little less than 19 hrs, say 18 hrs and 40 minutes, ( actually it's about 18:50 , but 18:40 is close enough.)
Some students find it difficult to make the conversion between star clock time and local daylight time, even when they have correctly identified star clock midnight for thier location and date. The prescription that the star clock moved backwards at half speed is not easy for many students to put into numbers. To alleviate this situation, I present the star clock calculator !
1. Follow the link and print a small ordinary 12 hour clock image that looks like this:
Cut out a circle around the wheel just leaving the numbers visible. This Wheel will represent star clock time
2. Follow the link and print a large counter clockwise 24 hour clock image that looks like this
The time shown on this clock represents local daylight time ( in 24 hour format, so 22 means 10 PM etc.)
3. Use a pin or staple or paper clip to fasten the centres of both clocks together ( the small star clock should be on top .)
4. Rotate the small wheel until the time identified as star clock midnight on the little clock lines up with the 0,24 hours on the big clock representing local daylight time.
the following image shows and example for which star clock midnight is about 2:30 ...
5. Now simply convert star clock and local daylight time back by reading star clock time from the inner clock and local daylight time from the outer clock
e.g. 5 O'Clock star clock time corresponds to about 19 hrs ( 7 PM ) Local Daylight time
2PM ( 14 hours ) local daylight time corresponds to about 7:30 star clock time.
This calculator is not guaranteed to get you within 15 minutes of the true local time ( you will get bonus marks if you acheive that level of accuracy .) It will give you times that are off by less than an hour ( that level of accuracy is required to earn full marks .)