The 13-sign zodiac places the Sun, Moon and planets in the actual constellations they stand in front of, at their real, unequal sizes. That adds Ophiuchus and changes where most of your chart falls.

If you have only ever seen twelve signs, this can feel like someone moved the furniture. So I will take it slowly, and I will tell you where it is solid and where it is a choice.

The idea in one paragraph

Most zodiacs cut the sky into twelve equal slices of 30 degrees. The 13-sign approach does not. It follows the constellation boundaries astronomers use, which the International Astronomical Union fixed around 1930. Some signs are wide, some are narrow, and there are thirteen of them. Where a planet is, in this system, is simply which constellation it is standing in front of.

The thirteen signs and the Sun's dates

Here are the dates the Sun spends in each one, as used in my chart engine. They can shift by a day either way from year to year.

  • Sagittarius: about 18 December to 20 January
  • Capricorn: about 21 January to 16 February
  • Aquarius: about 17 February to 11 March
  • Pisces: about 12 March to 18 April
  • Aries: about 19 April to 13 May
  • Taurus: about 14 May to 21 June
  • Gemini: about 22 June to 20 July
  • Cancer: about 21 July to 10 August
  • Leo: about 11 August to 16 September
  • Virgo: about 17 September to 30 October
  • Libra: about 31 October to 23 November
  • Scorpius: about 24 to 29 November
  • Ophiuchus: about 30 November to 17 December

Look at the spread. The Sun spends only six days in Scorpius and around six weeks in Virgo. That unevenness is the whole point of the system, and it is also the thing that makes some people love it and others distrust it. You can read more about the newcomer in Ophiuchus and the 13th sign.

What changes in your chart

Because the boundaries are the stars' and not the calendar's, your placements can move a long way. A planet you know as tropical Sagittarius may fall in Ophiuchus or Scorpius here. The rising sign changes too, and so does everything built on it.

For example, a Sun at 25 degrees of tropical Scorpio, which is around 17 November, sits in Libra in the 13-sign system, because the Sun does not reach Scorpius until about 24 November. That is a very different starting story, and neither is "the wrong one".

Houses in a 13-sign chart

If the signs number thirteen and houses are counted sign by sign from the rising sign, you get thirteen houses. I cover that in 13 houses in astrology, and I am open about the fact that there is no agreed standard for it.

Where it is solid, and where it is a choice

Solid: the constellations are real, the boundaries are an official astronomical standard, and the Sun genuinely passes through thirteen of them.

A choice: treating each constellation as a "sign" with its own meaning. Astrology has always assigned meaning to the sky, and a system that follows the constellations more literally is one way to do it. It is not the only way, and it is not the traditional way in either Western or Indian astrology. Those use twelve equal signs, and a great deal of careful thought has gone into them.

Common questions

Is it new? Constellation-based astrology has been discussed for decades. It is a minority approach, and it is not the standard in either Western or Indian astrology.

Is it more accurate? It is more literal about the sky. Whether that makes it more meaningful is a personal judgement, because astrology is not science. I do not call any system the correct one, and you should be wary of anyone who does.

Did NASA endorse it? No. That story is a persistent rumour, and I explain where it came from in the Ophiuchus post.

Should I switch? You do not have to. Try it as another lens on the same chart and see whether it says something useful.

How I use it

In my Solar Blueprint reading, Sidereal 13 is one of four skies you can choose as your main reading, alongside Tropical 12, Sidereal 12 and Vedic. I show the other three next to it, so you can compare your own placements and decide which language helps you reflect. The reading is written by me with my own tools, and I check each one before it reaches you.

If you would like to compare notes with other people who are curious about this, you are welcome in the ADHD MindFlow community. To understand why the twelve-sign and thirteen-sign zodiacs disagree in the first place, read sidereal vs tropical astrology.

Astrology is a way to reflect, not science, and it does not predict events.

Astrology is a way to reflect, not science, and it does not predict events. Astronomy facts here are checked against the sources linked. Read our sources, commercial disclosure and editorial approach.