Modern astronomers have recognized that the sun’s position on the ecliptic on any given date gradually moves westward. A natural phenomenon called precession of the equinoxes is responsible. This effect, known since 150 BCE, is caused by the moon (and sun and planets) pulling on the bulge of the earth’s equator. The Earth is 13 miles (21 km) wider at the equator than at the poles. This has moved all of our solar birthdays a month westward since the inception of astrology. The motion continues about 1 day westward every 72 years. Here is a list comparing the astrological dates with the astronomical dates. The astronomical dates tell us when the sun is really found in the constellation in modern times.

Constellation   Astrology                 number    Astronomy                number
or Sign            Dates                       of days     dates                        of days
Aries               21 Mar – 19 Apr         30           19 Apr – 30 May        38
Taurus            20 Apr – 20 May         31           14 May – 19 Jun        37
Gemini           21 May – 21 Jun         32           20 Jun – 20 Jul          31
Cancer           22 Jun – 22 Jul           31           21 Jul – 9 Aug           20
Leo                23 Jul – 22 Aug           31           10 Aug – 15 Sep       37
Virgo              23 Aug – 22 Sep         31           16 Sep – 30 Oct        45
Libra              23 Sep – 23 Oct          31           31 Oct – 22 Nov        23
Scorpius        24 Oct – 21 Nov          29           23 Nov – 29 Nov         7
Ophiuchus                                                        30 Nov – 17 Dec       18
Sagittarius     22 Nov – 21 Dec         30            18 Dec – 18 Jan       32
Capricornus   22 Dec – 19 Jan         29            19 Jan – 15 Feb       28
Aquarius        20 Jan – 18 Feb         30            16 Feb – 11 Mar       24/25
Pisces           19 Feb – 20 Mar         30/31        12 Mar – 18 Apr       38

As you can see, if your astral sign is Gemini, the sun was likely to have been in Taurus. From 21 May to 19 June, the sun was in Taurus on your so-called Gemini birthday. Geminis born on the 20 or 21 of June are also astronomically in Gemini. A similar pattern is true for all signs. For most people, the position of the sun (on a particular date) has moved one constellation to the west (to the right as you look southward) from the astrological positions of 2000 years ago. Also, you can see that for some people, the sun was in the constellation Ophiuchus on their birthday and not in Sagittarius as astrology would indicate.

The effect on the star of your birth is this: Over 50 years, the sun may be aligned with a different star on a given day. For example, in 1948 on July 13 the Star of your Birth would be Castor in the constellation Gemini. (Another star, 69 upsilon gem, is also possible, but Castor is much brighter so you would likely want it as the star.) In 2000, Castor is really aligned on the 14^th, and there is no star for the 13^th – but you could choose Castor again as the brightest star nearest to being aligned with the sun. Both years are leap years. This is important, and is covered on its own page.

The dates in the chart above are based on 2000 era coordinates. Be aware that leap year affects these dates, so this chart may vary by a day from other similar charts you may find.

There are a small number of astrologers who do take precession into account and who use Ophiuchus as a sign.

The other notable effect of precession is to change the direction where our North Pole points, thus changing which star acts as the North Star. The blue arrow shows the direction of this motion toward Cepheus.
By Tauʻolunga (Own work) [CC-BY-SA-2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons