Ascendant Calculation

So now we know what Sidereal Time is (if you don’t, read the Sidereal Time page first), we can use it to calculate the Ascendant.

Let’s start with a trivially simple example.

John Smith was born on 6 December 1970, in Greenwich, London, England at exactly midnight. What is his Ascendant and Midheaven?

I’ve chosen Greenwich deliberately, because (by definition) it has a longitude of 0W00 (or 0E00 if you prefer) - it’s the “Greenwich Meridian”. It’s completely arbitrary - whereas the equator is an obvious line on the Earth to measure our position north or south (you can see at a glance on a globe whether you live in the northern or southern hemisphere), measuring east or west is more arbitrary - east or west of what, exactly? A line was chosen to run through London - through the Royal Naval Observatory at Greenwich, to be precise - and this is the Greenwich Meridian. Whether you live in Sydney, Cape Town or New York, you always give your longitude as so many degrees east or west of this arbitrary line. The latitude of Greenwich, London is 51N32

Looking at a midnight ephemeris, we see the following:

The sidereal time at midnight on 6 December 1970 is 04:57. This is shown in the “ST” column.

In fact, the sidereal time at midnight on 6 December of any year is going to be around 04:57. Some ephemerides - the American Ephemeris, for instance - gives sidereal times to the nearest second, and these will differ slightly from year to year, but we’re really not interested in that level of accuracy.

As far as we’re concerned, the sidereal time is 04:57.

So - what’s the Ascendant and MC?

This is where we turn to a Table of Houses. There are various formats for these tables. The most famous set of tables is Raphael’s, which has a very old-fashioned feel to it, and which I find messy to use - however, they’re the most common set of tables, so we’ll look at those in a minute.

The sample on the left shows a section from a Table of Houses for 51 degrees North. To find out John Smith’s Ascendant, we simply look up 04:57 in the table and read off the house cusps accordingly. We don’t have 04:57, but 04:56 is close enough - so we have an Ascendant of 19 Virgo and an MC of 15 Gemini. We can also read the house cusps (Placidus) for the 11th, 12th, 2nd and 3rd houses. The other houses are of course simply the opposites - so the 11th house cusp is 22 Cancer, the 5th is therefore 22 Capricorn.

The Raphael’s tables give the same information. You find the relevant page - in this case, “Tables of Houses for London, Latitude 51 degrees 32 minutes North” and look up the nearest sidereal time. The time we’re interested in isn’t listed - but 04:55 (well, 04:54:52) and 04:59 are, so the position we want is going to be half way between the two. All positions are given to the nearest degree, except for the Ascendant, given to the nearest minute. In this case, we can see that the Ascendant is about 19 degrees of a sign. The sign is printed at the top (so under Ascendant we have the sign for Virgo), and is also printed when it changes. This is confusing - if, for instance, you look at the Ascendant for a sidereal time of 00:44, you can see it says 4 33. The symbol under “Ascendant” is Cancer. However, looking up the column, you notice that the sign changes from Cancer to Leo at 00:22 - so the actual Ascendant for a sidereal time of 00:44 is 4 degrees and 33 minutes of Leo, not Cancer. This is a trap into which many an unwary astrologer has fallen!

Now let’s make the examples progressively more complicated:

Peter Jones was also born at Greenwich, at 18:00 on 6 December 1970.

We’ve already determined that the sidereal time AT MIDNIGHT on 6 December 1970 was 04:57. 18:00 is eighteen hours after midnight, so we add on another eighteen hours to the sidereal time to get 22:57.

However, if we’re going to be fussy, we need to make a minor adjustment. Remember, the sidereal time slips by 4 minutes a day - so at midnight, the sidereal time in the ephemeris is exact. However, by noon the sidereal time will have slipped by two minutes, and by very late that night it will be out by almost four minutes. At 18:00, it’s going to be out by three minutes - so we need to ADD three minutes to the sidereal time, to give us 23:00.

This tiny adjustment is called “the acceleration on the interval”, and if you forget to do it, it’s not going to mess up your calculations too much. Forget the text book formula of adding “10 seconds per hour” - I just use this simple rule of thumb for a midnight ephemeris: if it’s the early hours of the morning, don’t bother with an adjustment; if it’s breakfast time, add a minute; lunch time, add two minutes; tea time, add three minutes; late at night, add four minutes. Now that’s MUCH simpler!

We then simply look up 23:00 in a table of houses for London, and get an Ascendant of 15 Cancer, and an MC of 14 Pisces.

Now let’s look at a more complicated example:

Susan Williams was born in Bristol, England on 6 December 1970 at 23:26GMT. Bristol is at 2W35, 51N27.

As before, we know the sidereal time at midnight on 6 December 1970 was 04:57. Susan was born at 23:26, so we add 23 hours 26 minutes to the sidereal time:

04:57 + 23:26 = 28:23 (remember, there are 60 minutes in an hour, not 100 - so this addition is a bit fiddly; if you have a calculator that handles hours and minutes, it makes life much easier!).

We also need to add on the minor adjustment: it’s late at night, so add on another four minutes:

28:23 + 0:04 = 28:27

However, Susan wasn’t born in London, at a convenient longitude of 0W00. This is significant. Before the days of trains, people would tell the time using a sundial. When the Sun was as high in the sky as it could be, that was noon. Astrology was simple. If Susan had been born in 1770 instead of 1970, and her birth time was given as “nearly half past eleven in the evening”, we would have known that meant 23:30ish local time - and the calculation we’ve done above would be the correct sidereal time to use.

However, the advent of trains meant that clocks had to be synchronised across the country. Bristol is west of London. That means that when the Sun is at its high point (noon) in London, it’s still late morning according to my sundial in Bristol. Bristol noon is about ten minutes later. So, when Susan tells me she was born at 23:26, she actually means 23:26 London time. She was actually born at 23:16 Bristol time - and since we’re interested in what sign was rising in her birth place when she was born (rather than whatever happened to be rising in London), we need to make another adjustment.

The adjustment is 4 minutes of time per degree of longitude. If you were born West of the Greenwich meridian, you SUBTRACT this number of minutes. If you were born East, you ADD this number.

So - Bristol is 2W36. The Sun moves at a rate of 4 minutes of time per degree, so 4 x 2 degrees 36 mins is about 10 minutes of time. Bristol is WEST of Greenwich, so we need to SUBTRACT 10 minutes - so her sidereal time is not 28:27, but 28:17.

Finally, we have the correct Local Sidereal Time - the thing we look up in the Table of Houses. However, the Table of Houses only runs from 00:00 to 23:59 - so whenever a sidereal time is more than 24:00 (or indeed less than 00:00, which it can be if we end up subtracting a large number), we need to either add 24 hours or subtract 24 hours. So, instead of looking up 28:17 in the Tables (which we can’t), we look up 04:17 in the tables instead. This gives her an Ascendant of 12 Virgo, and an MC of 6 Gemini.

Now let’s consider an example a long way from the Greenwich meridian:

Marilyn Munro was born in Los Angeles on 1 June 1926 at 09:30 PST. LA is 118W17, 34N00.

Now we have a problem of a different time zone. It was all very well all towns in Britain synchronising their clocks with London, but it would have been ludicrous for the USA to have done the same. When it’s noon in London, it’s noon GMT everywhere in the world. However, in New York, although it’s noon GMT, most New Yorkers are having their breakfast, and in Los Angeles they’re still in bed! So for this reason, time zones were born. The Sun is at its high point over London. The Sun takes 4 minutes to move a degree, so to move the 118 degrees to LA, the Sun will take about 118x4 = 472 minutes. That’s 7 hours and 52 minutes - or almost 8 hours. So, to make life easier, in LA they set their clocks to be 8 hours behind GMT. When the clock in London says it’s noon, the clock in LA says it’s 04:00. So, when we say Marilyn was born at 09:30PST, we need to convert that to GMT first - she was born at 09:30PST, which is 17:30 GMT.

We also have the very, very thorny problem of Daylight Saving Time. This has always been an absolute nightmare for astrologers, as each country (and in some cases each state within a country) has its own rules about when DST applies and when it doesn’t. Fortunately, modern software has a built-in atlas that tells you this - but even then, it doesn’t always get it right. However, the golden rule before calculating a chart is: CONVERT THE BIRTH TIME TO GMT. This will involve making an adjustment for Daylight Saving Time (also called Summer Time) where necessary, and converting the time zone of the local country to GMT.

So - the steps involved to calculate Marilyn Munro’s Ascendant and Midheaven are as follows:

1. Convert her birth time to GMT: 09:30 PST is 17:30 GMT

2. Find the sidereal time at MIDNIGHT on 1 June 1926 from an ephemeris: 16:35. Add this to the GMT time: 17:30 + 16:35 = 34:05

3. Now make the tiny adjustment - 17:30 GMT is around teatime, so add three minutes to get 34:08

4. Now adjust for longitude: 118 degrees 17 minutes times 4 is 473 minutes (the “cheat” way of doing this if you don’t have a calculator is: take the degrees part of longitude and multiply by 4: 118x4=472. Take the minutes of longitude, and divide by 15 - this gives 17/15 which is pretty close to 1. Add this 1 to the 472). That is 7 hours 53 minutes. LA is WEST of Greenwich, so SUBTRACT 07:53:

34:08 - 07:53 = 26:15

5. This is more than 24:00, so subtract 24 hours to get a sidereal time of 02:15.

6. Now look up this sidereal time in a Table of Houses for a latitude of 34 degrees North to get an Ascendant of 13 Leo, and an MC of 6 Taurus.

Now one final example, this time from the Southern Hemisphere. The details are similar to the Northern Hemisphere, but with a twist at the end:

Nelson Mandela was born in Umtata, South Africa, on 18 July 1918 at 14:54 EET. EET is 2 hours ahead of GMT. Umtata is at 28E42, 31S30.

1. Convert his birth time to GMT: 14:54 EET is 12:54 GMT.

2. Find the sidereal time at MIDNIGHT on 18 July 1918 from an ephemeris: 19:40. Add this to the GMT time: 12:54 + 19:40 = 32:34

3. Now make the tiny adjustment - 12:54 GMT is around lunchtime, so add two minutes to get 32:36

4. Now adjust for longitude: 28 degrees 42 minutes times 4 is 115 minutes (the “cheat” way of doing this if you don’t have a calculator is: take the degrees part of longitude and multiply by 4: 28x4=112. Take the minutes of longitude, and divide by 15 - this gives 42/15 which is pretty close to 3. Add this 3 to the 112). That is 1 hour 55 minutes. South Africa is EAST of Greenwich, so ADD 01:55:

32:36 + 01:55 = 34:31

5. Now here’s the twist - if it’s a Southern latitude, add 12 hours: 34:31 + 12:00 = 46:31

6. This is more than 24:00, so subtract 24 hours to get a sidereal time of 22:31.

7. Now look up this sidereal time in a Table of Houses for a latitude of 31 degrees 30 North (because you can’t usually get Southern Tables of Houses) to get an Ascendant of 24 Gemini, and an MC of 6 Pisces.

8. Now here’s twist number two - if it’s a Southern latitude, swap the signs round: So Nelson Mandela’s Ascendant is not 24 Gemini, but 24 Sagittarius; his MC is not 6 Pisces, but 6 Virgo.

If you use Microsoft Word, you may find the following files useful:

Chart.doc is a blank form that you can use to do this calculation - it has each step marked out

ChartMarilyn.doc is the worked example for Marilyn Munro

ChartMandela.doc is the worked example for Nelson Mandela

USING A NOON EPHEMERIS

All these examples assume you’re using a midnight ephemeris. It makes life so much easier if you do - since times begin at 00:00, you simply ADD the actual birth time to the sidereal time with a midnight ephemeris. With a noon ephemeris, it’s much more fiddly. If someone was born at 08:00, that’s four hours BEFORE noon so you need to remember to subtract 4 hours instead of adding 8 hours. Similarly, you need to remember to either add or subtract our tiny little adjustment - very messy.

The easy answer is: don’t bother with a noon ephemeris. However, if you’ve got a noon ephemeris already, don’t panic! There’s a very easy way to convert the Sidereal Time given in a noon ephemeris to the Sidereal Time at midnight. Simply do this, then proceed as in the above examples. So, to use our 6 December example - we’ve seen that the Sidereal Time at midnight on 6 December 1970 is 04:57. If you look up the Sidereal Time for 6 December 1970 in a noon ephemeris, you get 16:59.

Well, we know that Sidereal Time moves forward by one day and four extra minutes each day, so noon - which is 12 hours, or half a day, after midnight - sees Sidereal time move 12 hours and 2 minutes. So you simply subtract 12 hours and 2 minutes from 16:59 to get 04:57.

In fact, an even easier tip: do the 12 hours bit first. You can either add or subtract 12 hours (if it’s 3pm, then 12 hours ago it was 3am. In 12 hours time it will also be 3am), whichever is easiest. When you’ve done this, just subtract 2 minutes.

Examples:

Sidereal Time in a noon ephemeris for 14 February 2002 is 21:37. Easier to subtract 12 hours, to get 09:37. Then subtract the two minutes to get 09:35.

Sidereal Time in a noon ephemeris for 16 June 2001 is 05:39. Easier to add 12 hours in this case, to get 17:39. Then subtract the two minutes to get 17:37.

For further details, e-mail me at chris@bristolastrology.net or telephone me on 0117 963 6847