Time

Notes on Time

A discussion on Time:

The second is the fundamental unit of time. It was originally defined as: - the amount of time required for a 1-m pendulum to swing from one side of arc to the other

Now, it is defined as: 9 192 631 770 cycles of radiation corresponding to the transition between two hyperfine levels in the ground state of Cesium-133

Worth noting: There are approx 365 1/4 mean solar days in one solar year (watch time). The mean solar day is the average length of a solar day, 24 hours.

Solar year - the time between 2 vernal equinoxes. This is actually the tropical year, which is growing shorter by 0.5 sec/century. 1900 is the standard tropical year.

Sidereal time - time by the stars

Sidereal year - the amount of time for the Sun to return to a given position among stars

Calendars:

Julian - 365 days with an extra day every 4 years (leap year).
This was still a bit imprecise - consider that in 1988, the year was 365 d, 5 h, 48 min, 43.5 s. By 1582, the Julian calendar was out of phase with Easter by nearly 10 days. So, Pope Gregory XIII adopted a new calendar; 10 days were dropped from that year.

Gregorian calendar -
Years evenly divible by 4 are leap years. Every 4th century year is a leap year (2000, 2400; NOT 1600, 1700, 1800, 1900, 2100)

Daylight savings time
http://en.wikipedia.org/wiki/Daylight_saving_time
http://www.timeanddate.com/time/dst/

Changed a few years back. Now: DST Starts at 2 AM, second Sunday in March - set clocks AHEAD 1 hour
DST Ends at 2 AM, first Sunday in November - set clocks back (to standard time) 1 hour

We are EST, Eastern Standard Time. During DST, we become EDT (Eastern daylight time).

Greenwich Mean Time (GMT) - 5 hours ahead of EST. Roughly the same as Universal Time (UT).

Universal time (UT)
Basically the mean solar time as measured on the Greenwich meridian, thus, 5 hours ahead of us. Formally, UT is defined by a mathematical formula as a function of sidereal time and is thus determined by observations of stars, and kept via atomic clock

Sidereal time
In 365 1/4 solar days, Earth makes 366 1/4 rotations on its own axis. So, there are 366 1/4 sidereal days in a solar year. Each sidereal day is shorter by about 4 minutes than a solar day. UT and GST agree at one instant every year (at the autumnal equinox, around Sep 22). Thereafter, the difference between them grows, in the sense that ST runs faster than UT until exactly half a year later, when it is 12 hours. Another half-year later, the times again agree.

Greenwich Sidereal Time
local sidereal time on the Greenwich meridian

Julian Date (JD)
Jan 1, 4713 BC is the fundamental epoch from which this is decided. The Julian date is the number of days since this day.
There is no year 0 in astronomy. The year before 1 AD is defined as year 0. So, 10 BC is the year -9 in astronomy. That trick again: to go from BC year to astro year, subtract one and change sign.