**Local Solar time**This is what God gave us. The Sun transits (crosses the meridian) at noon, local time. This works just fine for farmers, but there are two difficulties for civilized man with quartz timepieces. The length of the Solar day (noon to noon) varies during the year due to the eccentricity of the terrestrial orbit. In addition, when your sundial reads noon, your neighbor's doesn't (unless it lies directly north or south of yours). Local time is a*local*phenomenon.**Civil Time**This is what the railroads standardized in the 19^{th}century, to overcome the problems associated with local time (Whose local time do you set your watch to?). Since the day is 24 hours long, the angular velocity of the Sun is 15^{o}per hour. Civil time consists of 24 time zones, each 15^{o}wide, centered on lines of longitude which are multiples of 15^{o}(there are local variations). The Eastern time zone is centered on 75^{o}west longitude. Noon EST occurs when the fictitious mean Sun crosses the 75^{th}degree of longitude. The fictitious mean Sun differs from the true Sun in that it has a constant angular velocity across the sky. The difference (in time) between the true Sun and the fictitious mean Sun, the Equation of Time, reaches nearly +/-15 minutes. The analemma on older globes demonstrates the Equation of Time.Standard time and daylight time are all parts of Civil time.

**Universal Time**is the zero-point of civil time. UT is the civil time at 0^{o}longitude (the standard meridian), which passes through Greenwich, England. UT is also known as Greenwich mean time (GMT), and is military time Zulu (Z). UT is based on the fictitious mean Sun.UT = 12

^{h}+ the Greenwhich hour angle (GHA) of the fictitious mean Sun.UT1 is UT corrected for the motion of the geographic poles (the Chandler wobble and similar phenomena).

UT2 is UT1 with an extrapolated correction for the spindown of the Earth.

UTC (Coordinated Universal time) is basically UT1, rounded off. Leap seconds are added to keep UTC within 0.9 sec of UT1. UTC is broadcast by WWV radio. You can see this time on the seismograph in the ESS lobby.

**Atomic Time**TAI is kept by atomic clocks. It is very accurate, and is not affected by the vagaries of solar system dynamics. 0:00:00 TAI = 0:00:00 UT2 1/1/1958.**Ephemeris Time**ET, like UT, is based on the mean fictitious Sun, but one with the angular velocity of the Sun on 1900 0.5 January. The right ascension of the fictitious mean Sun is given by the expression

RA=18^{h}38^{m}45.836^{s}+ 8,640,184.542^{s}T_{E}+ 0.0929^{s}T_{E}^{2}where T_{E}is the number of Julian centuries (36525 days) since 1900 0.5 January.

ET-UT = 24.349^{s}+ 72.318^{s}T_{E}+ 29.950^{s}T_{E}^{2}Ephemeris time was formally abolished in 1984, and was replaced with Terrestrial Dynamical Time (TDT) and Terrestrial Barycentric Time (TBT).

Because the JD is a large number, you will often see the Reduced Julian Date (JD-2,400,000) used. The modified Julian Date (MJD) is also commonly used. MJD=JD-2,400,000.5. Note the extra 0.5 day in the definition. Modified Julian dates increment at the same time as Universal Time. Some have been known to use HJD=JD-0.5. Be sure you know what convention is being used. Times in the Batten catalog are in Reduced Julian Days.

To convert the civil time into the Julian day, use the following
algorithm

where y is the year, m is the month, d is the day of the month,

f=y for m>2 and f=y-1 for m<3

g=m for m>2 and g=m+12 for m<3

A=2-fix(f/100) + fix(f/400).

When you compute with Julian dates, be sure to use double precision floating point arithmetic.

The IDL procedure **JULDATE** will return the reduced Julian date. In IDL,
type **juldate,jd**, and you will be prompted for input.

- 1
^{h}=15^{o}/cos() - 1
^{m}=15'/cos(), and - 1
^{s}=15"/cos()

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