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Ken Ward's Astrology Pages

Astrology: da Vinci (Julian Date)

clear.gif (807 bytes)   The purpose of this page is to point out that when doing a chart for someone in the past, a lot of careful research may be needed to determine the actual date of birth, or for other charts, the actual date.

For instance, Leonardo da Vinci was born on 15 April 1452, Florentine Calendar. Some say, at 9:40 pm. Based on his being born 3 hours into the night, with a 6:40 pm sunset. This is a Julian date. In the middle ages, the Florentine calendar was used in Italy. The new day began at sunset, not, as in our system, midnight. So someone born a few hours after sunset, would be born in the next day according to their recokoning, but for our reckoning, it would be the same day.

This means, therefore, that by our modern reckoning, that da Vinci was born on 14 April 1452, Julian Calendar.

Consequently, we take the following as his birth date in the old Julian Calendar: 14 April 1452 at 9:40pm.

This might suggest he was an Arian, but we must wait and see. We need to add 9 days to the date to turn it into our modern day Gregorian calendar. This means, in the Gregorian calendar, we have the date of birth as 23 April 1452 at 9:40 pm. The place was Vinci (10E55, 43N47). As I am going to use the Astrodienst tables, and I know they are in Julian for that year, the date will be kept to Julian (14 April 1452), because the conversions have already been done in the tables.

Calculate Greenwich Mean Time:

  1. No time zones. The date is LMT
  2. Birth time = 9:40 pm
    Time of birth=21:40 (24 hour-style)
  3. Subtract (East) longitude equivalent in time for 10E55
  4. Reduce by 24 hours, if necessary: 20:56:20
  5. This is the Greenwich Mean Time of birth, 14 April 1452, used to calculate the planets
  6. Rough check: as this is East, GMT will be earlier

Now calculate the Greenwich Sidereal Time of birth:

Because there are no time zones, the local mean time is the time we have, or 9:40pm.

  1. Greenwich Sidereal Time at midnight, 14 April 1452, Julian, is: 14:04:20
  2. Add LMT at midnight: 14:04:20+
  3. Reduce to within 24 hours: 11:44:20
  4. Add correction for GMT acceleration @10 secs/hour on 21:40:00
    210+7 secs, or 3min 37 sec:
  5. 11:47:57 is the Greenwich Sidereal Time at birth, which is used to calculate the House Cusps.
  6. The astrology program, which I wrote, gives the answer: 11 47' 49", astrodienst says 11:47:47. The reason for the difference is in part, a 4 second special correction, which hasn't been used above. This leaves up to 6 seconds unaccounted for.

He was born at 20:56:20, GMT, so the proportion of the day was approximately 20.9/24, or 0.871.


14 Apr 1452 2 47' 28" (taurus)
15 Apr 1452 3 45' 39" (taurus)
Daily movement 58' 11" (taurus)

Sun moved=
50' 45"

Hence position is:

2 47' 28"+50' 45"=
3 38'13"

Or about 3ta38


14 Apr 1452 22 aq 28
15 Apr 1452 4 pi 35
Daily movement 1207'

1207' x 0.871=1034'

So the moon was 3pi02


Vinci (10E55, 43N47), sidereal time: 11:47:57

From tables for ascendant

Time Lat 43 Lat 44 Differences Prop  
11: 44 6sa27 5sa49 -00:38 0.7833x-00:38=
11: 48 7sa14 6sa36 -00:39 0.7833x-00:39=
        Difference 00:46
      Difference 46x1=46 00:46
      Ascendant:   6sa44
      Ascendant (program) 6sa39

The difference between the two times in the tables is 4 minutes. The sidereal time of birth differs from the first value by 00:03:57, so the time factor=
3:57/4=1, which means the time is about 11:48, and the second time (11:44) isn't really needed.

The difference in latitudes is 1, and the latitude is 43N47, so the latitude factor is:
00:47/1= 0.7833 (00:47 is 0.7833 as a decimal)