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Ken Ward's Astrology Pages

Astrology: Rough Calculations (Obvious things about a chart)

clear.gif (807 bytes) As in all walks of life, it is easy, even for highly skilled people, to do something really silly. There are certain obvious things about a chart, which can be used to check that the chart is reasonably accurate. Some of these rough calculations are very rough, but they do produce a rough idea and they can identify major errors.

It should be routine to check the obvious about all charts. (Although in the age of machine calculation it might be LESS necessary).

It is also true, that even if you are talking to someone who knows only their birth date, you can reasonably guess certain things about their chart.

Sun Sign And Degree

When looking at a chart, check the sun sign accords with the birth date. The actual dates vary slightly by a day or two. But the last week of a month and the first three weeks of the next month tend to be in the same sign.

Below is a list of approximate dates for each of the twelve signs:

  1. Aries 21 March - 19 April
  2. Taurus 20 April - 20 May
  3. Gemini 21 May - 20 June
  4. Cancer 21 June - 22 July
  5. Leo 23 July -22 Aug
  6. Virgo 23 Aug. - 22 Sept
  7. Libra 23 Sept. - 22 Oct
  8. Scorpio 23 Oct. - 21 Nov
  9. Sagittarius 22 Nov. - Dec. 21
  10. Capricorn 22 Dec. - 19 Jan
  11. Aquarius 20 Jan. - 18 Feb
  12. Pisces 19 Feb. - 20 March

For instance, Aries is often March 21st to April 19th. So a person born between these dates will have their sun in Aries. If they were born on April 19th, clearly their sun would be about 29 degrees of Aries, see this page for conversion. If on March 21st, then we would expect about 0 degrees of Aries.

Actually a person born at 10pm on 20/mar/1990, for example, would be Aries. So this is approximate. But if a person born on 19th of April said they were Aries, it is believable, although perhaps they are Taurus? But they certainly aren't Pisces or Leo!

So the first guess or check is to ask whether the sign and the birth date are reasonable.

Do be careful of historical dates, as the date in the Julian Calendar is 11 or more days behind the date in the Gregorian Calendar. For example, Leonardo da Vinci was born on 15 April 1452, Florentine Calendar, which in Julian is 14 April 1452. Because we need to add some days (9), this means, he is a Taurian, and not Aries (our calendar equivalent 23 April 1452)!


Roughly, the degree of the sun is the number of days the sun has been in the sign.

For instance, if someone said they were born on 31st July, we would know they are Leo, and expect their degree to be 8 because:

  1. Leo is Leo 23 July -22 Aug, and
  2. 31st July is 8 days on from the 23 July

. Actually 31st July 1997 had the sun at 7degrees 50 minutes at the start, and 8 degrees 48 at the end of the day, but normally we would expect to be accurate within a degree or two.

Sun's House

The division of the houses will clearly depend on the House System used. For instance it will be different for Placidus (normal system) from the equal house system. The Houses changes about every two hours. This is accurate for the Equal House System only, and is a guess for Placidus and other systems.

The sun in the chart represents its position in the sky, although East in the chart is on the left, and West is on the right, opposite the normal!

sunHouse.gif (27623 Byte)

If the person is born about dawn, their sun will be near the first house cusp, and for a birth within 2 hours before dawn, the sun will be in the first house. So if dawn is 6am, then if they are born between 4 am and 6 am, their sun will be in the first house (approximately, if the Placidean Houses are used). If they are born after dawn, within 2 hours, they will have their sun in the twelfth house.

If they are born at midday, they will have their sun near the tenth house cusp (MC). And if they are born at sunset, their sun will be in the seventh house cusp.

If they are born at night, their sun will be in the lower part of the chart, and if in the day, in the upper part.

If they are born in the evening, then their sun will be in the Fifth House. If in the early morning, in the Second House.


If the sun is in Libra, that is she was born between Sept 23 and Oct 22, then at sunrise, the Ascendant will be about 0 degrees of Libra. So if she was born about 6 am, if this is dawn, then her Ascendant will be in Libra, or possibly Virgo.

Position of Mercury and Venus

Mercury is always in the same sign as the sun or in the neighbouring signs. So if the sun is in Gemini, then Mercury will be in Taurus, Gemini or Cancer. This is because Mercury is never more than 28 degrees from the sun. So if someone says they were a Gemini, and there Mercury is in Aries, you would be suspicious (it can't be), and if they said their Mercury was in Scorpio, you would know they were wrong.

Similarly, because Venus is never more than 48 degrees from the sun, it will always be with 2 signs of the sun. So if the sun is in Virgo, then Venus is in Virgo, Cancer, Leo, Libra or Scorpio. This doesn't tell us much, but it does tell is that a Virgo with Venus in Capricorn  has something wrong with the chart!

Retrograde Planets

The outer planets (Mars, Jupiter, Saturn, etc) are retrograde when they are opposite the sun in the chart. That is, in the opposite half of the chart. That is, within a sign or two of the opposite sign to the sun sign.

The inner planets are retrograde when they are between the sun and the earth, but this isn't detectable on the chart.

The "points", such as the moon's node, do not follow these rules.

We can conclude here that there is something wrong with a chart which shows the outer planets retrograde, when they are in the same half of the chart as the sun. Similarly, if they are opposite the sun (or within a sign of this), then we expect them to be retrograde.