Glossary of terms used on the Project Pluto WWW site
This portion of the Web site is obviously quite incomplete... as the need arises, phrases will be added to this section.
Delta-T. Delta-T is the difference, in seconds, between Dynamical Time and Universal Time. It's therefore also known as "TD-UT".
Delta-T basically reflects the difference between an "ideal" Earth that spins at a constant rate of exactly once per day relative to the Sun, and the "real" Earth that spins somewhat more erratically due to irregular tidal effects from the Moon. The major result of that tidal effect is that the Earth's rotation slows down gradually, so that the length of a day is gradually increasing. However, there are a lot of unpredictable speedups and slowdowns on top of that gradual increase, which makes the amount of Delta-T uncertain in the distant past and future.
Dynamical Time (TD). TD is the successor to the older "Ephemeris Time". In this time system, a "second" is a unit of constant length, so we have a uniform time scale. (In the Universal Time scale, in contrast, we're willing to adjust the length of a day so it will match the somewhat irregular motion of the earth.)
Various flavors of TD are of great importance in astronomical calculations. In these, it's essential that a "second" at one time match the length of a "second" at another time.
Julian Day (JD). The "traditional" calendrical systems, with uneven units such as months, days, and years, and months and years of unequal lengths, is not very handy for astronomical computations. Therefore, astronomers commonly reckon time according to the Julian Day system, which simply counts the elapsed days (and decimal fraction of a day) since noon, 1 January -4712, Universal Time. Based on this system, noon at 1 January 2000 was JD 2451545.0.
For astronomers, this system offers several advantages. If you want to know the number of days elapsed between two different dates, you can just subtract the JD of one instant from that of another. You'll very rarely deal with dates before the year -4712, so the JD value will almost always be positive. Conversion between "day, month, year" and JD values is a little cumbersome, but is explained in many reference works (in fact, it is part of the basic astronomical function library available on this Web site.) Time zones are no longer a worry.
Universal Time (UT). The time system usually used for timing astronomical events is Universal Time, also known as UT. This is essentially the civil time used at Greenwich; corrected for time zones, it is used throughout the world.
To convert from UT to your local time zone (in the US), subtract five hours to get EST, six to get CST, seven for MST, and eight for PST. For Daylight Savings time, subtract one hour less than this.
UT follows the actual, physical rotation of the Earth; the idea is to have one UT day match, as exactly as possible, one rotation of the earth relative to the Sun. This causes some troubles, because the Earth's rotation is somewhat irregular due to tidal effects from the moon. So one "real" rotation of the Earth is not going to be exactly 24 hours, or 1440 minutes, or 86400 seconds, long. In general, it will be a little more than this, and the amount of "extra" time will vary somewhat erratically. (In contrast, the Ephemeris Time scale disregards the rotation of the earth, so each day and second is of constant length.)
A little thought provides two possible solutions to the problem of irregular "days". You can re-define a "second" to be a little longer, so there will be exactly 86400 of them in a rotation of the earth. In this case, the length of a "second" will vary from one day to the next. This sort of Universal Time is called UT1.
The second solution is to leave the "second" as defined by atomic clocks: one "second" is the duration of 9 192 631 770 oscillations of a particular state of a cesium atom. In this case, since a particular rotation of the earth is going to be a little more than 86400 seconds, you will sometimes have to insert an extra second at the end of a day: a "leap second". This is exactly analogous to the way in which, since a year is a little over 365 days, we sometimes have to insert a "leap day" into the calendar. This sort of Universal Time is called UTC or Coordinated Universal Time, and is the "normal" sort of civil UT.
"Leap seconds" are always inserted at either 30 June 23:59:59 or on 31 December 23:59:59. Those days can therefore be 86401 seconds long instead of 86400 seconds long. Since the earth's rotation is irregular, the insertion of leap seconds is also irregular.