There are people born on the cusp of a sign who are often not sure which side of the cusp they should favor – the sign before, or the sign after. Often, they think they are half of one and half of the other. That’s just not true. You are one sign or the other, not both.
Using UT as frame of reference
I’ll present here a classic example of such uncertainty. This is the case of the “boss”, Bruce Springsteen, who was born on September 23, 1949 at 10:50 PM in Freehold, New Jersey. His birth date of September 23 appears to be right on the border between Virgo and Libra. Question is, is he a Virgo, or a Libra? Only one way to find out – bring out the good old Ephemeris! Here it is, for September of 1949, at UT 00:00. (A previous post, Ephemeris – Universal Time, described how the ephemeris lists positions for UT, which is time at zero longitude, and how we need to adjust the positions for local time.)
I am only showing a slice of the month, around his birth date.
The circled value is the position of the Sun on Sep 23, at midnight on zero longitude. If the Sun were to travel just another 22 minutes or arc, it would be at 0 degrees of Libra. The margin is razor thin, so we need to look at this very carefully.
What we really need to do is convert Springsteen’s birth date and time to the equivalent date and time at zero longitude. On Sep 23 in NU, daylight saving was on (it ended on Sep 25 in 1949). So to get the UT equivalent, we need to add 4 hours to 10:50 PM, which would make it UT 2:50 AM on Sep 24! But by that time the Sun had already moved into Libra – 00Li37 at UT 00:00 of Sep 24 – so if cast the horoscope for Springsteen, the Sun would definitely show in Libra.
Using Sun’s rate of motion
What if we know someone who born in NJ on the same day as Springsteen, but at an earlier time? Would their Sun be in Virgo or Libra? It gets trickier. Working backward from UT 00:00 of Sep 24 with Sun at 00Li37, the equivalent time at NJ is 8pm on Sep 23. In other words, as seen from NJ at 8pm local time, the Sun is at 00Li37. So anyone born at 8pm in NJ on Sep 23 would still have Sun in Libra. So the big question is, at what time does the cutoff happen?
The Sun has to move back 37 degrees of arc, to be exactly at 00Li00. (All of astrology works from the geocentric perspective, with the Earth as the fixed frame of reference. So when we say the Sun is moving, we mean it to be relative to the fixed earth.) How much time would the Sun need to get there? The Sun moves approximately 1 degree of arc per day. We know this because it traverses the entire 360 degree span of the sky in a year, or 365/366 days. Turns out the speed is not entirely uniform–during the summer months, the Sun moves a little slower (a little less than 1 degree per day), and in the winter months it compensates by moving a little faster.
But let’s go with 1 degree for our approximation (at worst we’ll only be off by 1 minute of arc). 1 degree is 60 minutes of arc, which is traveled by the Sun in 24 hours. So to travel 37 minutes of arc, it would take about 15 hours. Ok, so we said the Sun was at 00Li37 at 8pm of Sep 23 in NJ. Back 15 hours means 5am. So someone born at 5am would be smack dab on the border. If that person you know was born at 4 am or earlier, you could definitely say the Sun was in Virgo. 6am and later, definitely in Libra. But if their birth time was between 4am and 6am, you would have to get an astrology program to cast the horoscope to be certain. J
Just out of curiosity, I went ahead and cast a horoscope in Lifespan for 5am, and found the Sun to be exactly at 00Li00! At 4:30 am, however, it showed 29Vi59! (So the super hot zone is somewhere between 4:30 am and 5am.)
Using Moon’s rate of motion
If you have the stomach for this kind of thing, or indeed a burning desire to do more math like this in your head, you can try your hand at a similar process to deduce the Moon’s position, given the date, place, and approximate time of birth.
At the very least, you can find out whether the Moon is definitely in one sign or the other, or move on to other things if it happens to be at the border and don’t have enough birth information (place/exact time) to get more accuracy. All you need is the Moon’s rate of motion. What we know is that the Moon’s orbit around the Earth takes around 28-29 days. If we round this to 30 days, we have 360 degrees of arc traveled in 30 days, or 12 degrees in a day, or 1 degree per 2 hours.
This is much more granular compared to the Sun! In practice, this means even with the Ephemeris handy, it is much trickier to accurately get at someone’s Moon sign, if you don’t know their birth place and time–the Moon just moves so much faster.
So if you are up to the challenge, see if you can do some mental gymnastics to find the Moon sign for a person born on Jan 3, 1990, at 10:30 PM (standard time) in Sydney, Australia!