or: Sliding blocks of time
Each year, the Sun, as seen from Earth, travels along the line of the ecliptic, through the constellations of the zodiac; and also through the “signs” that have the same names. But not on the same dates: in general, the Sun enters a sign (such as Aries) some days before entering the sane-named constellation.
The reason is precession, which needs explaining. Precession is not something we are born with an understanding of!
Here are the dates for 2020, in the form of a list and the prettier form of a picture. The calendar dates change slightly in other years.
Jan 20 Mon 9 UT Sun enters Capricornus, at longitude 299.74° on the ecliptic.
Jan 20 Mon 15 Sun enters astrological sign Aquarius, i.e. its longitude is 300°.
Feb 17 Mon 3 Sun enters Aquarius, at longitude 327.91° on the ecliptic.
Feb 19 Wed 5 Sun enters astrological sign Pisces, i.e. its longitude is 330°.
Mar 11 Wed 17 Sun enters Pisces, at longitude 351.60° on the ecliptic.
Mar 20 Fri 4 Sun enters (at the equinox) astrological sign Aries, i.e. its longitude is 0°.
Apr 18 SAT 17 Sun enters Aries, at longitude 29.11° on the ecliptic.
Apr 19 SUN 15 Sun enters astrological sign Taurus, i.e. its longitude is 30°.
May 13 Wed 20 Sun enters Taurus, at longitude 53.49° on the ecliptic.
May 20 Wed 14 Sun enters astrological sign Gemini, i.e. its longitude is 60°.
Jun 20 SAT 22 Sun enters (at the solstice) astrological sign Cancer, i.e. its longitude is 90°.
Jun 21 SUN 9 Sun enters Gemini, at longitude 90.46° on the ecliptic.
Jul 20 Mon 13 Sun enters Cancer, at longitude 118.28° on the ecliptic.
Jul 22 Wed 9 Sun enters astrological sign Leo, i.e. its longitude is 120°.
Aug 10 Mon 9 Sun enters Leo, at longitude 138.21° on the ecliptic.
Aug 22 SAT 16 Sun enters astrological sign Virgo, i.e. its longitude is 150°.
Sep 16 Wed 14 Sun enters Virgo, at longitude 174.18° on the ecliptic.
Sep 22 Tue 14 2 Sun enters (at the equinox) astrological sign Libra, i.e. its longitude is 180°.
Oct 22 Thu 23 Sun enters astrological sign Scorpius, i.e. its longitude is 210°.
Oct 30 Fri 19 Sun enters Libra, at longitude 217.83° on the ecliptic.
Nov 21 SAT 21 Sun enters astrological sign Sagittarius, i.e. its longitude is 240°.
Nov 23 Mon 0 Sun enters Scorpius, at longitude 241.16° on the ecliptic.
Nov 29 SUN 20 Sun enters Ophiuchus, at longitude 248.06° on the ecliptic.
Dec 18 Fri 2 Sun enters Sagittarius, at longitude 266.63° on the ecliptic.
Dec 21 Mon 10 Sun enters (at the solstice) astrological sign Capricornus, i.e. its longitude is 270°.
What is the difference between constellations and signs?
The constellations were perceived groups of naked-eye stars, and the signs were originally the twelve of these along the ecliptic. But the signs were defined in relation to a zero point: the vernal equinox point, the point where the Sun appears at the March or northern-spring equinox. Around two millennia ago, the Sun entered the Aries area at that date (which is why the vernal equinox point is still traditionally also called “the First Point of Aries”), the Taurus area about a month later; and so on. So the “signs” are the twelve 30°-wide sections of the ecliptic that start at 0°, 30°, 60°, and so on, from the vernal equinox point.
Constellations, all over the sky, had no [recise boundaries, or curving ones drawn to taste by map-makers, until fixed ones were agreed by the International Astronomical Union in 1930. And these straight-line boundaries are quite complicated; and those that cross the ecliptic do not do so at regular 30° intervals. Aries, for instance, is quite narrow; Virgo is wide. And there is a striking exception: most of Scorpius is south of the ecliptic, a stretch of which is instead in Ophiuchus, not traditionally a member of the zodiac.
Still, around two millennia ago, there was approximate correspondence: the Sun was in Aries in the month starting at the March equinox, in Taurus the next month, and so on. But precession was lying in wait.
Precession (there is a large section on it in my Astronomical Companion) is a slow wobble of the Earth’s spin axis, over a cycle of 25,800 years. The ecliptic stays where it is, but the equator – the plane in which Earth spins – steadily changes the direction of its tilt. Therefor the vernal equinox point, which is the point where the equator crosses the ecliptic, keeps shifting westward along the ecliptic. Two millennia ago, it was at the beginning (western edge) of Aries. But by now it has slid back into the middle of Pisces.
Therefore the points 30°, 60]d, and so on, along the ecliptic, which are called the beginnings of the signs Taurus, Gemini, and so on, are now well west of those actual stars, and the Sun reaches them before it reaches those stars.
The dates when the signs are reached stay about the same, except for the one-day variation caused by leap days. That includes the equinoxes and solstices of late March, June, September, and December, those being when the Sun reaches ecliptic 0°, 90°, 180°, and 270°. But the dates of reaching the astronomical constellation boundaries became a little later each year.
The Sun always enters sign Aries on or about March 20. But h
Here for comparison are the dates when it entered Aries (as now defined) in various years, with the longitudes where the (now defined) Aries boundary then was:
-500 Mar 20 17:51 Sun enters Aries 354.10
-500 Mar 26 14:58 Sun enters sign Aries 359
-500 Mar 20 17:51 Sun enters Aries 354.10
-500 Mar 26 14:58 Sun enters sign Aries 359
1 Mar 24 3:44 Sun enters Aries 1.03
500 Mar 27 1:33 Sun enters Aries 7.94
1000 Mar 30 5:31 Sun enters Aries 14.89
1500 Apr 2 9: 7 Sun enters Aries 21.86
2000 Apr 18 13:39 Sun enters Aries 28.83
2020 Apr 18 16:45 Sun enters Aries 29.11 29.535
And here, to make it all clearer, we hope, or at least more colorful, are “sliding blocks of time” diagrams for a current year and about a thousand years ago,
The sign-times stay where they are. (As do the equinoxes and solstices, denoted by the thick black lines.) The constellation-times get later, slowly enough that you’ll notice no difference for next year, and very little difference for next decade. But in 25,800 years they will slide all around.
The Taurus-Gemini crossing
The longitude of this constellation-boundary-point (continually changed by precession like all points along the ecliptic) was 600 at about 160 BC and has now passed 900 – the northernmost point on the ecliptic, where the Sun is at the north-hemisphere summer solstice.
This happened, according to the calculation of Jean Meeus, on 1990 Jan. 1. In 2017, 2020, and 2024, it reached 90.40°, 90.46°, and 90.50°.
This beginning-longitude of Gemini is now roughly what that of the next constellation, Cancer, anciently was, and thus what that of the astrological sign Cancer still is. From 1990 onward, the Sun has not quite left Taurus when astrology says it is entering Cancer, two constellations ahead.