# The Answer Box

## Mark Pottenger

Deborah: Why do we use Standard time (Greenwich Mean Time) rather than true local time or sidereal time to calculate the planets (as we do the house cusps with sidereal time)?

Mark: The basic answer to the question is that it is cheaper to do it the way we are now doing it.

The subjects involved in explaining that answer are rather complicated, so my explanation here will necessarily be incomplete. The article on astronomy in the Sagittarius 1977 issue goes into more detail on parts of it. Ephemerides are tables of planetary positions given at regular intervals. The interval is usually as long as is compatible with accuracy, thus some ephemerides for slower objects have intervals up to 10 days. (Heliocentric ephemerides, because they are dealing with much more even motion, can have even longer intervals between entries.) The interval between entries in most astrological ephemerides is one day. The time of day for which positions are given is usually midnight (0 hours—the beginning of the day) or noon (12 hours—the middle of the day). Older ephemerides are labeled as noon or midnight Greenwich Mean (now Universal) Time, which is a particular standard time.

We use a standard time rather than a local time to avoid extreme expense and annoyance. A local time is just that—the solar (or sidereal) time for a specific longitude on earth. Local mean time is local time in which the variations in the sun’s apparent daily motion (caused by the earth’s elliptical orbit) have been averaged out. (Local apparent time is what a sundial will show you.) Standard times are special cases of local times—the local mean time of the zone meridian. Universal Time is the local mean time (or the zone time) for the earth’s zero longitude meridian. By using a single standard time for ephemerides no matter where on earth a person is actually concerned with, we avoid unnecessary duplication. If we were to calculate and print an ephemeris for each time zone in the world, we would end up with 24 ephemerides for every 1 we have now.

(Actually, a few more would be needed to take care of half hour zones like India and a few even odder zones.) If we produced an ephemeris for the local mean time of every even degree of longitude on the earth, we would have 360 ephemerides for every 1 we have now. If we produced an ephemeris for the local mean time of every even minute of longitude on the earth, we would have 21,600 (360x60) ephemerides for every 1 we have now. You can see that Local Mean Time Ephemerides would be a bit impractical and expensive. The only ephemerides I am aware of which don’t use Ephemeris or Universal time are some calendars which give positions in terms of the time in a particular zone (e.g., Eastern or Pacific).

As for using Sidereal Time for an ephemeris, it could be done only with difficulty. There would be no problem using Greenwich Sidereal Time instead of Greenwich Mean (Solar) Time to set up an ephemeris, but there would be dating problems. Because the sidereal day is 4 minutes shorter than the solar day, any given solar (calendar) day has a 4 minute span of sidereal time that occurs twice. This could cause considerable confusion.

The time for which positions are given in modern ephemerides is actually called Ephemeris Time. For most purposes, the effect of the difference between Universal and Ephemeris times is negligible, but for really precise work it should he taken into account. Universal (Greenwich Mean) Time is the mean solar time at the earth’s prime meridian of longitude running through Greenwich Observatory in England. Mean solar time is defined by the earth’s rotation with respect to the sun. In this century our measurements of time have become accurate enough for us to discover that the rotation of the earth is not perfectly uniform. (Over a long time span, the rotation of the earth is slowing down. This means that a day now is longer than a day a century ago. On top of the general slowing trend, there are minor seasonal and other short term variations in the speed of the earth’s rotation.) Thus, Universal (or any other rotational) Time is subject to slight changes. For purposes of astronomical calculations, a measure of time which is absolutely uniform is needed—so we now have Ephemeris Time. Ephemeris Time differs from Universal Time by less than one minute for any date in this century so far. (The difference is already 50 seconds, and will probably reach a full minute before the end of the century.) The difference between Ephemeris and Universal Time, called delta T, has to be determined after the fact—the whole reason for establishing Ephemeris Time in the first place was that the variations in the earth-tied Universal Time were unpredictable.

Ephemerides, because they are dealing with planetary positions, are dealing with phenomena that never repeat exactly. Thus, an ephemeris always needs both date and time. Tables of Houses do deal with phenomena that repeat exactly, so one Table of Houses is good indefinitely. House geometry is strictly dependent on sidereal time and latitude—date is irrelevant (unless you are extremely picky, in which case the slow change in the obliquity of the ecliptic will make date relevant again). You do need a date to get an accurate sidereal time, so in that sense it is still relevant. Any sidereal time (or any solar time) is for a specific longitude on earth, so you must specify what location the time is for. The first entry in ephemerides after the date is the Greenwich Sidereal Time for the solar midnight or noon for which the ephemeris gives positions. (Note: the GST is given for 0 or 12 UT—not ET—so delta T is not taken into account in calculating sidereal times.) The sidereal time at Greenwich at the moment of birth is simply the GST (for 0 or 12 hours) from the ephemeris plus the Universal (solar) Time interval since 0 or 12 hours plus the difference between that interval of solar time and the equivalent interval of sidereal time (see a basic natal math article or book if you want more details). Once we know the Sidereal Time in Greenwich at the moment of birth, the Sidereal Time at the place of birth (or any other place) is just a longitude correction away. Convert the longitude of birth from degree-minute-second to hour-minute-second (time) measure by dividing by 15 and subtract this from the GST (add if east of Greenwich) to get the Local Sidereal Time. Because you use the exact longitude of birth, this time is truly a Local time.

The only reason you would ever need more than one Table of Houses is if you use or test more than one system of houses (Placidus, Koch, Regiomontanus, Campanus, Meridian, etc.). So long as your table of houses covers one full sidereal day of 24 sidereal hours and covers an adequate range of latitudes you don’t have to keep buying a new one every decade like ephemerides.