Introduction

To do serious astronomy one must have a good understanding of time.

Time is simple, right? Actually time is not simple at all, we are just used to it. Take for example there is no zero on clocks. Sure there is a ten which is a 10, but that is ten, not zero. Older clocks use the Roman Numeral X for ten. There is no Roman Numeral for zero. Now some may say zero is 00:00:00 hours which is midnight. Indeed, some modern time uses a 24 hour system and it starts at midnight at 00:00:00 hours. Military or Zulu time is like this and in astronomy, Universal Time (UT). All these 24 hour times are based on the time at that instant on the Greenwich Meridian (GMT) and start at midnight equal to 00:00:00 hours. Some digital clocks allow local time to also be 24 hour based and not based on GMT, but local midnight. When I say 00:00:00 hours I mean hh:mm:ss, where hh are two digits for the hour (06), mm are two digits for minutes (13) and ss for seconds (33.874564). You can make the time as accurate and precise as you wish by adding the numbers to the right of the decimal point in the seconds part. Time is now 06:13:33.874564 (06 hours, 13 minutes and 33.874564) seconds).

Types of Time

Local Standard Time
Local Daylight Savings Time
Universal Time (UT)
Sidereal Time
Local Sidereal Time
Julian Date (JD)
Heliocentric Julian Date (HJD)
Modified Julian Date (MJD)
Reduced Julian Date (RJD)

Local Standard Time

This is the time in a particular time zone. Time zones are divided up for convenience and sometimes very confusing. Slightly North, South, East or West and it could be a different hour.

In the continental United States we have 4 time zones Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST) and Pacific Standard Time (PST) There are similar time zones around the world. Based on UT EST is 5 hours behind. When UT is 23:00:00 hours EST is (-5 hours) 18:00:00 hours, CST is (-6 hours) 17:00:00, MST is (-7 hours) 16:00:00 and PST is (-8 hours) 15:00:00.

Things get confusing when say EST is 20:00:00 as it is 01:00:00 hours UT the next day. In astronomy it is always good to supply a double date where the first date is the evening and the second is the next morning's date.

If you make an observation on the evening of 29 December 2009 at 18:00:00 hours EST that would be reported as 29/30 December 2009 23:00:00 UT. Both the UT and Local date would be 29 December.

If you make an observation on the evening of 29 December 2009 at 20:00:00 hours EST that would be reported as 29/30 December 2009 01:00:00 UT. The UT date would be 30 December, but the Local date 29 December.

Local Daylight Savings Time

In the continental USA the time zones then become EDT (UT - 4), CDT (UT -5), MDT (UT - 6) and PDT (UT -7). While this may seem simple, it is not. Not everywhere uses Daylight Time. In fact most, but not all, of Arizona never uses Daylight Time so during the months daylight time is in effect, most of Arizona is still on Standard Time (MST) and thus PDT is the same as MST. With proper adjustments all is well. Note: Universal Time does not change with Daylight Savings Time.

Universal Time (Also Known a UTC or Coordinated Universal Time)

In astronomy, most of the time people of observing events outside of the Earth. It behooves us to find a standard whereby astronomical events can use the same time. This is where Universal Time or UT comes in. It is the same as Greenwich Mean Time defined as 12:00:00 hours when the Sun transits the zero longitude line that passes through Greenwich, England. While this can vary by up to 16 minutes (due to the equation of time or variation of the Earth's elliptic speed and axial tilt), UT is now set by an atomic clock (UTC). Generally UTC and UT are the same time.

Sidereal Time

The Earth is divided up by lines of latitude going North and South of the Equator and lines of Longitude going East and West of the GMT meridian. From our perspective on Earth, the sky can be thought of as a sphere and divided up similarly.

Instead of lines of Latitude and Longitude, the celestial sphere has lines of Declination or Dec (latitude) and Right Ascension or RA (Longitude). The celestial Equator is above the Earth's Equator. As the Earth rotates on its axis the celestial sphere appears to turn from East to West. The Declination is just like Latitude going from zero degrees at the Equation to + 90 degrees at the North Pole and -90 degrees at the South Pole. Longitude or Right Ascension is divided up into 24 hours instead degrees.

Because of the Earth's motion around the Sun a sidereal day is slightly less than 24 hours. A sidereal day is 23 hours 56 minutes and 04.091 seconds. The zero sidereal hour is a meridian on the celestial sphere where the Sun crosses the celestial Equator in March. This is also known as the first point of Aries and Vernal Equinox. The RA of a Star is it's sidereal time.

When that RA time of a star is the same as the local sidereal time, that star is on your meridian. The local time shifts about 2 hours a month for the sidereal time. In other words a star is on your meridian at 11:00:00 hours UT (04:00:00 hours MST) on 15 September. The star will be on the meridian at 09:00:00 hours UT ( 02:00:00) MST) ion 15 October and 07:00:00 hours UT (00:00:00 hours MST) in November.

Local Sidereal Time

Local Sidereal Time is just the RA that is on your meridian at a given time. Epsilon Aurigae has a RA of 05 hours, 02 minutes. When the star is on your meridian, your Local Sidereal Time is 05:02.

Julian Date (JD)
Also known as Geocentric Julian Date (Referenced to the center of the Earth)

Astronomical events that occur over the period of months and years become difficult to track the time accurately due to the variation on the length of months and years. Julian Date is a means to track evens accurately over thousands of years event though there are leap years and calendars have changed. Julian Date is one number for the day with a decimal point to allow accurate representation of the time of day as accurate as one wishes.

Where did Julian Date come from? Contrary to some beliefs, Julian Date or Day has no connection with the Julian Calendar and was not named after Julius Caesar. So where did it originate? In 1583, Joseph Justus Scaliger developed the Julian Period. Scaliger took three cycles, the 28-year solar cycle, the 19-year lunar cycle, and the 15-year cycle of the Roman Indiction (used in calculating Easter) and multiplied them together. The resulting period (28 X 19 X 15 = 7980) is 7980 years. He then set about to determine the last time all three of these cycles passed through zero. It turned out to be the year 4713 BC.

This is a very convenient time because all of recorded history has happened since then. All recorded astronomical events of interest occurred after this date. Astronomers found this to be a very useful reference and by using it, times between events were independent of day-of-the-week, month, or year.

One somewhat confusing issue is that JD starts at noon UT not at midnight. Thus on 30 September 2009 at 11:30 UT the JD is 2,455,104.9583, but at UT= 12:30 one hour later the JD is 2,455,105.0417.

The decimal part of JD can be calculated by just dividing the decimal hour by 12 remembering that after UT= 12 and before the next UT day you must subtract 12 from the time. For UT 09.4537 the JD decimal part is just 09.4537/12 or 0.7878.

An easy way to determine the Julian Date day is to reference to a recent day that you know the Julian Date for. For example 01 October 2009 is Julian Date 2,455,105. Then 10 October 2009 would just need 9 days added (2,455,114). The part of the day is referenced to 12:00 UT.

For additional information about Julian Date refer to the following:

Reese, R.L., Everett, S.M., and Craun, E.D., THE ORIGIN OF THE JULIAN PERIOD: AN APPLICATION OF CONGRUENCES AND THE CHINESE REMAINDER THEOREM, Am J. Phys. 49 (7), July 1981, 658-661.

Moyer, G., THE ORIGIN OF THE JULIAN DAY SYSTEM, SKY and TELESCOPE, April 1981, 311-313.

Heliocentric Julian Date (HJD)

Because an event in a distant star system can appear to be timed differently (up to 16 minutes - time for light to travel across the Earth's orbit around the Sun), a time based on the center of the Sun is desired. For astronomical photoelectric photometry the use of Heliocentric Julian Date allows precise timing information. Heliocentric Julian Date (HJD) is the Julian Date referenced to the center of the Sun and can be found using the following equations:

HJD = JD (Geo) + Hel Corr

The Hel Corr (Heliocentric Correction) can be determined by:

Hel Corr = T * R * (COS (L) * COS (A) * COS (D)) + T * R * (SIN (L) * (SIN (E) * SIN (D) + COS (E) * COS (D) * SIN (A)))

Where:

T = Light travel time for one astronomical unit (499 seconds or 0.0057755 days)
R = Earth -Sun distance in astronomical units L = Longitude of the sun
A = Star's right Ascension (in decimal hours)
E = Obliquity of the ecliptic = 23.45 degrees
D = Star's declination (in degrees decimal)

R and L must be found from the AMERICAN EPHEMERIS AND NAUTICAL ALMANAC for each observing night. For those who do not wish to get into the math, there are web sites that will calculate this for you. For example: http://aa.usno.navy.mil/data/docs/JulianDate.php

Because of the long period of the epsilon Aurigae eclipse, HJD is not needed. Just JD will work fine.

Modified Julian Date (MJD)

Because space is sometimes limited, particularly on plots, it is sometimes advisable to use a reduced or modified Julian Date. The MJD is some part of the JD, usually the last 4 or 5 digits plus a 0.5 day.

MJD = 55,096.4965
where
JD = MJD + 2,400,000.5

Reduced Julian Date (RJD)

Like the MJD the ides here is to reduce the number of common digits, but unlike MJD the added factor does not have the 0.5 day added. For example, JD = 2,455,096.9965, the RJD would be

RJD = 55,096.9965
where
JD = MJD + 2,400,000
or
RJD= 5,096.9965
where
JD = MJD + 2,450,000

depending on whether or not you want to include the 50,000 unit. In the case of epsilon Aurigae, the 1982-1984 eclipse had similar last 4 digits of the JD so by adding the fifth, confusion can be avoided.

1982 - 1984 eclipse
RDJ = 45,096.9965
JD = RJD + 2,400,000.0

2009 - 2011 eclipse
RJD = 55,096.9965.
JD = RJD + 2,400,000.0

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