AAVSO: Manual for Visual Observing of Variable Stars

Variable star observations reported to the AAVSO should always be expressed in terms of the Julian Day (JD) and the decimal part of the day given in Greenwich Mean Astronomical Time (GMAT). This is the standard unit of time used by astronomers because it is convenient and unambiguous. Here are the advantages:

The astronomical day runs from noon to noon so that you don't have to change calendar dates in the middle of the night.

A single number represents days, months, years, hours, and minutes.

Data on the same star from people observing anywhere in the world can be compared easily since they are all relative to the same time zone; that of the prime meridian in Greenwich, England.

What follows is a simple procedure for figuring the JD and GMAT decimal of your observations.

Step-by-Step Instructions

Record the astronomical time and date of your observation as counted from local noon. Use the 24-hour clock instead of AM or PM.
examples:
A. June 3 at 9:34 PM = June 3 at 9:34
B. June 4 at 4:16 AM = June 3 at 16:16
Note that the date of your observation does not change after midnight because an astronomical day runs from noon to noon not midnight to midnight.

If your observation was made when Daylight Savings Time (Summer Time) is in effect where you live, subtract one hour to get standard time.
A. June 3 at 9:34 DST = June 3 at 8:34
B. June 3 at 16:16 DST = June 3 at 15:16

Figure the Julian Date equivalent to the astronomical calendar date of your observation as determined in Step 1 above using the JD calendar shown here.
A and B: June 3, 2000 = 2,451,699

Find the decimal equivalent of the hours and minutes of your observation from Table 4.1 and add the result to the JD integer found above. Notice that this table also takes into account your longitude (and thus time zone) so that the end result is expressed in GMAT.
Click here for Table 4.1 - JD Decimal of the Day - This chart can be used to convert into a tenth of a day, the time at which an observation is made, expressed in Greenwich Mean Astronomical Time.
Using the excerpt from Table 4.1 below, you can see that if you observed from the 15°E time zone, the GMAT decimal equivalent of 8:34 is .3. For the observation made at 15:16, it would be .6.
Click here for the excerpt from Table 4.1 (JD Decimal of the Day).
Now add the decimal to the JD integer determined in Step 3 to arrive at the final result of:
A. JD = 2453525.3
B. JD = 2453525.6

In the following section are several examples of conversion from local time to JD/GMAT. It is recommended that you work through each case until you feel very comfortable with the procedure. Remember that recording the proper date and time for each of your observations is absolutely essential!

Sample Calculations

Example 1 — Observation from Cambridge, MA, USA (75°W time zone) at 9:40 pm Eastern Daylight Time, June 22, 2005

Step 1: astronomical time = 9:40, June 22, 2005
Step 2: 9:40 - 1 = 8:40 on June 22, 2005
Step 3: JD = 2,453,544
Step 4: GMAT decimal = .6
Final result: 2,453,544.6

Example 2 — Observation from Tokyo, Japan (135°E) at 1:15 am, January 10, 2005

Step 1: astronomical time = 13:15, Jan. 9, 2005 Step 2: N/A
Step 3: JD = 2,453,380
Step 4: GMAT decimal = .2
Final Result: 2,453,380.2

Example 3 — Observation from Vancouver, BC Canada (120°W) at 5:21 am, February 14, 2005

Step 1: astronomical time = 17:21, Feb. 13, 2005 Step 2: N/A
Step 3: JD = 2,453,415
Step 4: GMAT decimal = 1.1 (add 1 day)
Final Result: 2,453,416.1

Example 4 — Observation from Auckland, New Zealand (180°E) at 8:25 pm, April 28, 2005

Step 1: astronomical time = 8:25, Apr.28, 2005
Step 2: N/A
Step 3: JD = 2,453,489
Step 4: GMAT decimal = -0.9 (subtract 1 day)
Final Result: 2,453,488.9

Note that as shown in example 4, if the time you observe is exactly the same as a time listed in Table 4.1, you should choose the larger of the two decimals.

Where does JD come from?
In the Julian Day system, all days are numbered consecutively from Julian Day zero, which began at noon on January 1, 4713 BC. Joseph Justus Scaliger, a French classical scholar of the 16th century, determined this as the date on which three important cycles coincided; the 28-year solar cycle, the 19-year lunar cycle, and the 15-year cycle of tax assessment called the "Roman Indiction".

The calendar seen here is representative of the one mailed out to AAVSO observers each year. It gives the last four digits of the Julian Day for each day of every month of the year 2005 (On the actual calendar, the months July-December are on the reverse side). For the complete JD, add 2,450,000 to the four digit value given in the calendar for the Astronomical Day of your observation.

Some observers prefer to create their own computer programs or use existing ones to calculate JD. See the AAVSO website for links to JD computing programs. (http://www.aavso.org/observing/aids/jdcalendar.shtml)

UT, GMT, and GMAT

Often in astronomy you will see the time of events being expressed in Universal Time (or UT). This is the same as Greenwich Mean Time (GMT) which starts at midnight in Greenwich, England. To find the UT equivalent of a specific time, simply add to it, or subtract from it, as the case may be, the zone difference for your observing location. The "World Map of Time Zones" (Figure 4.2) is provided to help you to determine the zone difference for your location. To convert from UT to Greenwich Mean Astronomical Time (GMAT) subtract 12 hours.

Two additional reference tables are provided in this chapter for your convenience:

Table 4.2 lists the JDs for the zero day of each month from 1996 to 2025. The zero day (which is actually the last day of the previous month) is used for ease in calculating the JD of any given day by making it possible to simply add the calendar date to the JD listed.
example: Jan. 28, 2005
Click here to see Table 4.2 (the list of JD numbers)
Table 4.3 can be used to find the GMAT decimal of the day to four decimal places. This degree of accuracy is only needed for certain types of stars (see Table 6.1, Chapter 6).
Table 4.3 - Click here to see Table 4.3, "JD Decimal (to four places) To use this table, find the GMAT hours across the top of the page and the minutes down the side. The result is the fraction of the day represented.

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