Once you’ve calculated the Ascendant and house cusps, finding planet positions is the easy bit! You’ll need an ephemeris, and The American Ephemeris is probably the best one. Don’t be put off by the title if you don’t live in the USA - some students assume that the word “American” means that it’s only valid for locations in the USA, or is set for USA time zones. It isn’t - it’s simply an American publication, and uses the extremely accurate data from NASA’s Jet Propulsion Laboratory. Below is a sample from the American Ephemeris for December 1970.

Again, the ephemeris so accurate you don’t want to get bogged down in the detail. For instance, it gives the position of the Sun to the nearest second. So - how accurate do you want your data? If I’m doing a chart by hand, I often only work to the nearest degree - in which case you can simply read the planet positions for everything except the Moon directly from the page in the ephemeris, regardless of birth time. However, if you want to know planet positions to the nearest minute, you will need to take birth time into account.

The ephemeris is based for midnight GMT. This is important. If Maria Lopez was born in Los Angeles at 22:00 PST on 6 December 1970, that means she was born at 06:00 GMT on 7 December 1970. Let’s see how we’d work out her planet positions:

Sun: the Sun moves about a degree a day. The ephemeris gives the position of the Sun at midnight on each day - at midnight on 7 December (remember, Maria was born on 7 December GMT, not 6 December) the Sun was at 14 degrees 28 minutes of Sagittarius (I’m rounding up seconds here). At midnight on 8 December, the Sun was at 15 degrees 29 minutes of Sagittarius. Maria was born at 06:00 GMT, which is a quarter of the way through the day. The Sun has moved 1 degree and 1 minute - that’s 61 minutes - in an entire day. So in a quarter of a day, it will have moved a quarter of 61 minutes, which is 15 minutes (rounding off very slightly). So, Maria’s Sun will be at 14 deg 28 plus 15 minutes, which is 14 deg 43 of Sagittarius.

OK - now that was a very easy example, because Maria happened to be born exactly one-quarter of the way through a day. Suppose she’d been born at 15:25 GMT? Now we have to do some serious number crunching, but let’s take it a step at a time - and if you really hate numbers, there’s another technique you can use instead: a graph. Let’s look at the graph first, as it helps clarify what we’re trying to do here.

At midnight on 7 December, the Sun was at 14 degrees 28 of Sagitarrius, so we put a blue dot on the vertical “midnight” line at about 14 degrees 28 (each square up is 5 minutes, so we may be a minute or two out here). At midnight on 8 December, the Sun was at 15 degrees 29, so we put another blue dot on the other midnight line. Then we draw a line connecting the two. This represents the path of the Sun during the course of 7 December.

So, to see where the Sun was at 15:25 GMT, we simply draw a line up from 15:25 (again, we can only be accurate to within a few minutes here) until it hits the blue line - then read off on the left to see how far the Sun has progressed. We get an answer of approximately 15 degrees 6 minutes (each square up is 5 minutes, and it’s a tad above that line). So, using this method, we get an answer of approximately 15 degrees 6 minutes of Sagittarius. We may be a minute or two out, though.

Now let’s have a look at how we do this numerically - you will need to use this method if you really want positions to the nearest minute. The other reason that things can get very confusing is the unfortunate use of the word “minute” which, of course, has two meanings - a sixtieth of an hour of time, or a sixtieth of a degree. To avoid confusion, I’m going to use the word Mins to mean minutes of time, and minutes to mean minutes of arc (a sixtieth of a degree).

Find out how far the Sun moves during the course of the day, in minutes. At midnight on 7 December, the Sun is at 14 degrees 28. At midnight on 8 December, the Sun is at 15 degrees 29:

15deg29 - 14deg28 = 1deg01, which is 61 minutes.

Now see how many minutes the Sun moves in a Min (of time). Well, in 24 hours, it moves 61 minutes. 24 hours is 1440 Mins. So the Sun moves 61/1440 = 0.042361 minutes per Min.

Now calculate how many Mins have passed between midnight and the birth time:

15:25 is 15 hours and 25 Mins after midnight, which is (15 x 60) + 25 = 925 Mins

So, from midnight to 15:25, the Sun must have moved 0.042361 * 925 = 39.2 minutes. Call this 39 minutes.

So, at midnight on 7 December, the Sun was at 14 degrees 28 minutes. Add 39 minutes to this to get 15 degrees 7 minutes of Sagittarius - this is the correct answer. You can see our graphical estimate of 15 degrees 6 minutes was pretty close!

You can deal with the other planets in the same way - Saturn moves about a minute a day, so you really don’t need to bother doing it for the outer planets.

The Moon is dealt with in a similar fashion - except that the American Ephemeris gives two positions for the Moon - midnight and noon. So, to use the same calculation to discover Maria’s Moon at 15:25 GMT, proceed as follows:

15:25 is 3 hours and 25 Mins after Noon. At Noon on 7 December, the Moon was at 6 degrees 25 minutes of Aries. At Midnight on 8 December, the Moon was at 13 degrees 26 minutes of Aries. So in 12 hours, the Moon has moved:

13deg26 - 6deg25 = 7deg01, which is 421 minutes. So, in 12 hours, which is 720 Mins, the Moon has moved 421 minutes. So it moves 421/720=0.58472 minutes per Min.

Maria was born at 15:25, which is 3 hours and 25 Mins after Noon, or (3 x 60) + 25 = 205 Mins

So, from noon to 15:25, the Sun must have moved 0.58472 * 205 = 119.9 minutes. Call this 120 minutes, which is exactly 2 degrees.

So, at noon on 7 December, the Moon was at 6 degrees 25 of Aries. At 15:25, it will have moved exactly 2 degrees, so is at 8 degrees 25 of Aries.