Dates and Times

Astronomers use a different time system than mere mortals. Amateur astronomers need to use these time systems too.

As I've said before, use the local time when making entries in your log. Converting your local date and time to one of the astronomical systems should be done after you're finished observing. Your full attention should be focused on accurate observations.

Universal time (UT) is used by all astronomers. UT is simply the time in Greenwich, England. Instead of a 12-hour clock, a 24-hour clock is used. UT allows astronomers from around the world to have a common reference for time. If we all know that something astronomical is going to happen at 22:43 hrs UT, we simply need to convert it to local time. Where I live, we are six or seven hours behind UT, depending upon whether daylight savings time is in effect, so I simply subtract the correct number of hours from UT to determine local time for me. When I convert local time to UT, I just add the correct number of hours. After you've used this system for awhile, it's second nature.

The Julian date (JD) is used by variable-star observers and was designed by Joseph Justus Scaliger in 1582 and has nothing to do with the Julian calendar. Scaliger named his system in honor of his father, Julius Caesar Scaliger.

This system enables variable-star observers to more easily compare the characteristics of stars over periods of years. The first Julian day begins at noon in Greenwich on January 1, 4713 BC. This date was selected because it happened to mark the start of three independent cycles of solar and lunar phenomena.

You're probably wondering if you need to figure the number of days since 4713 BC. No, you don't. A list of Julian days, essentially a calendar, is available from the major variable star organizations. For your convenience, noon on 1 January 2002, is Julian day 2452276.0 It's easy to set up a list of Julian days using a spreadsheet. Remember, the Julian day begins at noon in Greenwich; in other words, 1200 hrs UT.

The Julian day also allows you to indicate the time of day when you record or report an observation. Notice the decimal position for the Julian day in the last paragraph. Usually, the type of variable star being v50Pr*P«"a,ionS ^

observed dictates the accuracy, that's the number of decimal places that you need to record. For example, if you are observing an LPV, such as R Lep (Hind's Crimson star) with a period of 427d, then simply recording the day of your observation is sufficiently accurate. One day out of 427 is an accuracy of 0.2%. On the other hand, nearby S Eri, an RRc variable, with a period of 0^273 (6 hrs, 33 min, 6 sec) needs to be observed much more frequently and each observation must be reported with a greater precision than is required for R Lep. Since S Eri's period is accurate to a thousandth of a day (3 decimal places) you must report your observations with at least the same accuracy.

A calculator will help or you can set up a spreadsheet to help you. I find it easy to work with seconds when it comes to short intervals of time. If you multiply the number of hours in a day by 60 you can calculate the number of minutes in a day. The answer is 1440. If you now multiply the number of minutes in a day by 60 you can calculate the number of seconds in a day. The answer is 86,400. Now you can see that S Eri's period is 0.273 times 86,400, or 23,587.2 seconds. You can easily convert this to hours and minutes.

When it comes time to calculate the decimal time portion of the JD, perhaps the moment of your observation, follow a few simple procedures. For example, if your observation is at 11:34 PM, first convert the time to UT. Let's say that UT is six hours ahead of you. Simply add six hours to your local time. The time of your observation, 11:34 PM local time, is 0534 hrs UT.

Remember, the JD starts, and ends, at noon UT. Let's say that your observation was made on January 2, 2002. The JD would then be 2452277.0. Now refer back to the UT time of your observation. Since the JD begins and ends at noon, 12 hours must be added to the UT. So, 0534 plus 1200 equals 1734 hrs JD. Convert this to seconds by multiplying the 17 hours by 60 and then by 60 again. The answer is 61,200. Then multiply the 34 minutes by 60. The answer is 2040. Add 61,200 and 2040 for the total seconds. The answer is 63,240. Divide this by 86,400 to get the decimal time. The answer is 0.73194. Round your answer to an appropriate number.

Any questions on why you should be doing this after you have finished observing?

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