Astronomy Essentials

Constellations and signs: what’s the difference?

Chart illustrating constellations and signs.
View larger. | Constellations and signs – what is the difference? Here is the sun (below the horizon) at its May 20. 2020, position. On that day, the sun entered the astrological “sign” of Gemini. Yet, as you can see from this chart, the actual constellation Gemini is still above the horizon when the sun is below it. The sun didn’t enter the constellation Gemini until July, 2020. Chart via Guy Ottewell’s blog.

Guy Ottewell published this post on May 19, 2020, under the title The Difference Made by 2 Thousand Years. Reprinted here with permission.

Constellations and astrological signs

On Saturday, May 21, 2022, at 1 UTC, the sun enters the astrological sign of Gemini. This gives us a rather good way of seeing the difference between astrological signs and constellations as defined by astronomers. The sun doesn’t actually cross in to the actual constellation of Gemini until June 21, 2022, at 8 UTC.

Map showing difference of signs and constellations
Here is a star chart at the precise time the sun enters the astrological sign of Gemini on May 21, 2022 at 1 UTC. As you can see the sun – shown here as the bright yellow star-like object – is actually astronomically in the constellation of Taurus rather than Gemini. Chart via Heavens-above.com.

The signs of Aries, Taurus, etc. – still used in astrology – are 30°-wide bands along the ecliptic, starting at longitude 0°. This is also known as the First Point of Aries. The constellations are areas of the starry sky, defined since 1930 by specific lines and boundaries. The two coincided, somewhat more than 2,000 years ago, when the system of astrological signs was defined. But precession – the wobbling of Earth’s spin axis over a cycle of 25,800 years – has made them increasingly divergent.

The sun’s path through the sky

The chart below shows the sun’s travel from March 20, 2020, (the spring or vernal equinox) to May 20, 2020. You can see that the sun does indeed reach longitude 60° on the ecliptic. But this brings it to the beginning (roughly) of constellation Taurus, not Gemini. It will have to travel another 30° – one month – to enter Gemini.

Chart with ecliptic line going through formal boundaries of constellations, equator also marked.
View larger. | Chart showing the sun’s movement through the constellations, as defined by astronomers. You can see that sun won’t enter Gemini until July, 2020. Chart via Guy Ottewell’s blog.

The stars and constellations stay fixed. What shifts over time is the celestial equator – the “belt,” you could say, of the spinning Earth – and the mapping system based on it.

Picturing constellations and signs

Mentally move them. Imagine the sun’s March-to-May track, and the celestial equator – the two features I’ve emphasized with red on the chart above – slid 30° to the left (east), while everything else stays in place. The crossing-point of equator and ecliptic – which is the zero point for longitude – is 30° to the left: it is at what is now longitude 30°, the beginning of Aries. So it really is then the First Point of Aries. In this mental projection, the sun is at the First Point of Aries in March, and arrives at the gates of Gemini at this time in May.

This was how things stood when the system of signs was agreed, around 2,000 years ago.

You can, with some imagination, see it in your sky, or on the chart at the top of this page.

There is the sun (below the horizon) at its May 20, 2020, position where it enters the astrological sign of Gemini. If this were 150 BC it would be 30° on, at what is now longitude 90° – the solstice point of our time, by the feet of Gemini.

Read more from Guy Ottewell.

Bottom line: When – astrologically – the sun enters the “sign” of Gemini, it is still nearly one month away from entering the constellation Gemini in the sky.

Check here to see what constellation the sun is in today.

Check here to see where the sun is today in its astrological sign.

Posted 
June 1, 2022
 in 
Astronomy Essentials

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