Zodiac Sign Calculator

Overview

Finding a Western sun sign from a birth date needs no software — it is a lookup against twelve conventional date ranges. This page publishes those ranges, and explains why the boundaries move by a day either way between years.

• The sign boundaries are the moments the Sun crosses each 30-degree division of the tropical zodiac.
• Boundary dates shift by about a day between years because the tropical year is not a whole number of days.
• 'Cusp' births are births near a boundary — the Sun is in one sign or the other, never both.
• These ranges are astrological convention and do not match the constellation the Sun is actually in.

The conventional date ranges

• Aries — about 21 March to 19 April. Taurus — about 20 April to 20 May.
• Gemini — about 21 May to 20 June. Cancer — about 21 June to 22 July.
• Leo — about 23 July to 22 August. Virgo — about 23 August to 22 September.
• Libra — about 23 September to 22 October. Scorpio — about 23 October to 21 November.
• Sagittarius — about 22 November to 21 December. Capricorn — about 22 December to 19 January.
• Aquarius — about 20 January to 18 February. Pisces — about 19 February to 20 March.

Why the boundaries move

Each boundary is the instant the Sun reaches a multiple of 30 degrees of ecliptic longitude measured from the March equinox. That is a precise astronomical moment, not a calendar date.

Because the tropical year is about 365.2422 days rather than a whole number, that instant falls roughly six hours later each year and jumps back when a leap day intervenes. Over the four-year leap cycle the corresponding calendar date can therefore shift by about a day, which is why published ranges use approximate dates.

Births near a boundary

Popular writing describes people born close to a boundary as being 'on the cusp', sometimes suggesting they share both signs. Astronomically that is not the case: at any instant the Sun's longitude falls in exactly one 30-degree division.

For a birth within a day of a boundary, determining the sign requires the actual ingress time for that specific year and the birth time. This is one of the few cases where a sun-sign answer genuinely needs more than the date — and where a lookup table is insufficient rather than merely approximate.

This is not the Sun's constellation

These ranges belong to the tropical zodiac, anchored to the March equinox. Precession has carried that reference point about 30 degrees westward against the stars over roughly two thousand years, so for most dates the Sun is physically in the constellation preceding the astrological sign.

Sidereal traditions such as Jyotisha use star-referenced boundaries and therefore give different dates entirely. Neither system is miscalculating; they are anchored to different reference points, and each is internally consistent.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Zodiac signsThe twelve signs, elements, and modalities.
• ConstellationsThe astronomical regions the Sun actually crosses.
• Celestial eventsEquinoxes, solstices, and why they are instants.

Frequently asked

Why do sun-sign dates vary by a day between sources?

Because the boundaries are astronomical instants, not calendar dates. Each is the moment the Sun reaches a multiple of 30 degrees of ecliptic longitude, and because the tropical year is about 365.2422 days, that moment drifts roughly six hours later each year and resets at leap days. The corresponding calendar date therefore shifts by about a day across the leap cycle.

What does it mean to be born on the cusp?

Only that the birth fell close to a sign boundary. At any given instant the Sun's ecliptic longitude lies in exactly one sign — there is no astronomical sense in which it occupies two. Determining the sign for a birth within a day of a boundary requires that year's exact ingress time and the birth time.

Is my sun sign the constellation the Sun was in?

Usually not. These ranges use the tropical zodiac, measured from the March equinox, and precession has shifted that reference point about one full sign against the stars over two millennia. For most dates the Sun is physically in the constellation preceding the astrological sign, and it also crosses Ophiuchus, which is not a sign at all.