Before you can calculate a chart, you’ll need the following information about the person you’re doing the chart for:

Date of birth

This is the easy bit! Practically everyone knows their date of birth, and this is usually sufficient to work out which signs every planet is in. Very occasionally, you’ll find someone where you can’t quite work out their Sun sign - although newspaper columns may say Sagittarius is 22 November to 21 December, this isn’t exact. Someone born on 22 December 2002 in the early hours of the morning will still be a Sagittarius, as the sign doesn’t change from Sagittarius to Capricorn until 01:14 in the morning London time. For someone born in San Francisco, they could be born on 21 December 2002 and be an early Capricorn. The actual time that signs change varies from year to year, so it’s always a good idea to construct a birth chart for the actual date of birth. Because astrologers work with all planets, and are interested in the angles that planets make to each other, it isn’t sufficient to know that you are “a Pisces”. Astrologers need to know not just which sign each planet is in, but “how many degrees” of each sign. Remember, a “sign” is simply a segment of sky; the entire 360 degree circle of the sky is divided into twelve zodiac signs of 30 degrees each, so your Sun might be positioned a third of the way through the segment of sky called “Pisces” - so an astrologer would say your Sun is at 10 degrees of Pisces.

Most of the planets move slowly enough that just knowing the date of birth is usually reasonably accurate for most planets. The Moon (which isn’t technically a planet, but both the Sun and Moon are referred to as “planets” in astrology), however, moves about twelve degrees a day so frequently you won’t know your Moon sign unless you also know your time of birth. You certainly need the time of birth to know how many degrees of a sign your Moon is in.

One very important note if you share chart information with other astrologers: ALWAYS give the month as a name, not a number, and the year as four digits: 3 April 1960 is an unambiguous date. 3/4/60 probably means 3 April 1960 but could mean March 4 to someone in the USA, and of course it could mean 1860 or 1760 for a historical character.

Time of birth

Not everyone knows their time of birth, and without it you can construct a chart, but it won’t be complete. The time of birth is needed to determine your Moon position accurately, and is certainly needed to work out your Ascendant (or “rising sign”). The Ascendant is the segment of sky which is rising over the eastern horizon at the time of your birth - and because the Earth revolves once every day, all twelve zodiac signs rise in a 24 hour period. It is, of course, essential to know when someone says they were born at “eight o’clock” to know whether they mean eight in the morning or the evening. I always use the 24 hour clock, as do most astrologers - so a birth time of eight in the evening is written as 20:00.

Birth times are recorded on your birth certificate in most European countries, and in Scotland - but not in England or Wales. In the USA, times are recorded in some states but not all of them. For twins, times are always recorded on the birth certificate.

Time zones and Daylight Saving Time

This can be a problem for astrologers, since many people don’t necessarily know what time zone they were born in, or whether Daylight Saving Time (“Summer Time”) was in operation.

Time zones are a relatively recent invention - 150 years ago, people weren’t as aware of time as we are now, and would measure it based on the position of the Sun. If the Sun was directly overhead, it was “noon” and that was it. However, because the Sun takes time to make its apparent journey through the sky, “true noon” occurs at different times in different places. For instance, in England 150 years ago, when it was noon in London, it was about 11:50 in Bristol. Ten minutes later, the Sun was directly overhead in Bristol. It wouldn’t be directly overhead in New York until about five hours later. (Of course, now we standardise our clocks, when it’s 12:00 in London, it’s 12:00 in Bristol too, but it’s still true that when the Sun is directly overhead in London, it won’t be directly overhead in Bristol for another ten minutes). Astrologers use something called “Universal Time”, which is effectively the same as “Greenwich Mean Time (GMT)”. When the Sun is directly overhead in Greenwich, London it is 12:00 noon GMT. Most other European countries are an hour “ahead” of GMT - so when it’s noon in London, it’s 13:00 in Paris. Large countries like the USA have several time zones - New York is on “Eastern Standard Time” which is five hours behind GMT, whereas San Francisco is on “Pacific Standard Time” which is eight hours behind GMT. So when it’s noon in London, it’s 07:00 in New York, and 04:00 in San Francisco.

The situation is further complicated by Summer Time, or Daylight Saving Time. This is when clocks go forward by an hour in the summer. Astrologers still need to know what the “real” time is - the fact that we choose to put our clocks forward in the summer in Britain doesn’t mean that the planets suddenly jump at this time! If someone is born at 15:00 in June, British Summer Time, an astrologer will still need to know that this means 14:00 GMT. The reason Summer Time is so complicated is that the rules regarding when clocks go forward and backwards vary from year to year, country to country. For instance, in Britain clocks went forwards two hours during some years in the Second World War (from 1941 to 1945), then only an hour in 1946, then two hours in 1947.  Then in the early 1970s we had permanent Summer Time (even in the winter), but then reverted to the usual rules (forward in March, back in October). Even the rule about which week in October the clocks go back can cause confusion. In the UK some books say it’s the fourth Sunday in October, others say it’s the last Sunday in October. This is usually the same, but a few years ago when there were five Sundays in October, a lot of printed diaries got the date wrong! It’s hardly surprising that a lot of people don’t know whether Summer Time was in operation when they were born.

Fortunately, with the advent of the Internet, this confusion is a thing of the past. If you look at the Online Charts section, I’ve given a link to an excellent site that has a huge database of cities and time zone changes - it will work out all the necessary details for you.

Place of birth

Your place of birth doesn’t affect the position of the planets (other than the time zone you’re in, which is covered above), but it does affect your Ascendant. This is because the segment of sky that appears to be rising depends on where you are on the Earth. Any position on Earth can be defined by two co-ordinates: your latitude and your longitude. Latitude indicates how far north or south of the equator you are - so anywhere on the equator is 0 degrees North (or indeed 0 degrees South!), and the North Pole is 90 degrees North. Bristol is just over 51 degrees North. Astrologers like to be a bit more accurate than this, and a degree is divided into sixty minutes (not to be confused with minutes of time). So Bristol is actually 51 degrees 27 minutes North of the equator, usually written as 51N27. New York is 40N43, Berlin is 52N30, and Sydney in Australia is 33S52.

The equator is an obvious starting point for measuring latitude, but longitude presents more of a problem for determining how far east or west you are - east or west of where? An arbitrary line is chosen running through the Greenwich Observatory in London, called the “Greenwich Meridian”. Since Greenwich is actually in East London, Central London is at a longitude of 0W19, Bristol is 2W36, Berlin is 13E22, New York is 74W00 and Sydney is 151E13. Notice that latitudes go from 0 to 90 degrees, longitudes from 0 to 180 degrees. 180W is exactly the same as 180E (it’s the opposite side of the world to Greenwich). Again, most people know where they were born, but don’t know the latitude and longitude. Astrologers used to have to buy a special world atlas containing latitudes and longitudes of all the major cities, but again the Internet comes to the rescue. The database I referred to above, on the site in the Online Charts section, has a list of major cities and the exact latitude and longitude.