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Normal Stars

There are so many stars in the sky – how can we know which ones to pay attention to when observing conjunctions of the planets with the stars? Well, it turns out that conjunctions with the planets can only happen for a star that’s close to the ecliptic, the path that the planets and Sun and moon all follow as they travel across the sky day after day and year after year. This eliminates the majority of the stars in the sky from ever being found in conjunction with any of the planets.

And then, if we follow the ecliptic all the way around the sky, we actually find only a few stars whose placement and brightness might enable them to participate in observable conjunctions with the planets. These stars on or near the path of the ecliptic are referred to as normal stars. To narrow down which of these normal stars are most likely to be of interest, we’ll examine them to see which ones are brightest and closest to the ecliptic.

The path of the Sun itself best defines the ecliptic. Looking for stars of magnitude 2 and brighter (brighter being a lower number) and within 6 degrees of the ecliptic, as we track the Sun all the way around, for the year 2000 AD we find only the following stars in its path:

  • Regulus, in the constellation Leo, magnitude 1.4 and 0d 28m from the ecliptic
  • Spica, in the constellation Virgo, magnitude 1.1 and 2d 3m from the ecliptic
  • Antares, in the constellation Scorpius, magnitude 1.1 and 4d 34m from the ecliptic
  • Aldebaran, in the constellation Taurus, magnitude 1.0 and 5d 28m from the ecliptic
  • Elnath, in the constellation Taurus, magnitude 1.7 and 5d 23m from the ecliptic

Three of these candidates – Spica, Antares, and Aldebaran – are brighter than Regulus, so the distance of 6 degrees was chosen in order to be able to consider them. But looking again, we have to admit that neither Aldebaran nor Antares is what we could call “close” to the ecliptic. With the exception of  Venus, it would probably be rare for a planet to stray far enough from the ecliptic for a close conjunction with either of these two stars. So tightening up the restrictions a bit and looking at stars of magnitude 2 and brighter within just 4 degrees of the ecliptic, for the year 2000 AD we get:

  • Regulus, in the constellation Leo, magnitude 1.4 and 0d 28m from the ecliptic
  • Spica, in the constellation Virgo, magnitude 1.1 and 2d 3m from the ecliptic

This has eliminated three of the five from consideration, leaving only Regulus and Spica. These measurements do gradually change over time, so looking at the same thing for the year 2000 BC we get:

  • Regulus, in the constellation Leo, magnitude 1.4 and 0d 15m from the ecliptic
  • Spica, in the constellation Virgo, magnitude 1.0 and 1d 50m from the ecliptic

And for 4000 BC[1] we get:

  • Regulus, in the constellation Leo, magnitude 1.4 and 0d 5m from the ecliptic
  • Spica, in the constellation Virgo, magnitude 1.0 and 1d 47m from the ecliptic

So there was a time, long, long ago when Regulus was very close to the ecliptic. Though it’s shifted some in 6000 years, from almost right on the ecliptic to about half a degree away, it’s still much closer than the next closest candidate Spica – about 4 times as close. Regulus is unique. Over this entire 6000 year time span, of all the stars in the sky with magnitude 2 and brighter, Regulus is the only one found within a half degree (30m) of the ecliptic.

If we go back far enough, there’s a point where Regulus crosses over the ecliptic and for a short time is precisely on it. This zero point is found to be about 4970 BC. The following chart shows how the position of Regulus changes throughout history – after the zero point, getting gradually farther and farther from the ecliptic, up to almost a half degree in our present time:

5000 BC 0m 12s
4970 BC 0m 0s
4000 BC 5m 30s
3000 BC 10m 38s
2000 BC  15m 16s
1000 BC  19m 19s
1 BC  22m 47s
1000 AD  25m 39s
2000 AD  27m 54s
This is why it’s of interest to pay attention to Regulus and its conjunctions with the planets. It really is a special star, with a special place in the sky. It isn’t necessary to look to Astrology or to tradition for a rationale. All that needs to be considered are the purely astronomical measurements of brightness and closeness to the ecliptic.

[1] Note that planetarium programs typically lose some accuracy for dates as ancient as 4000 BC. In order to achieve more accurate results for ancient dates, the data shown for the position of Regulus has been taken from Solex (see http://www.solexorb.it/SolexOld/), which does a better job of reaching this far into the past.


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