As the sun appears to move through the sky, it traces a path – rising in the east (roughly), round to the south (if you live in the northern hemisphere), setting in the west (roughly). This path of the sun through the sky is called the ecliptic. At night, when you look at the stars, there are of course constellations all around – most people in the northern hemisphere will recognise The Plough (or Big Dipper in the US), and probably Orion. In the southern hemisphere, most people will recognise the Southern Cross, and also Orion.

This diagram (which shows just a small segment of the night sky) shows some familiar constellations: Orion, towards the bottom of the picture with the famous “Orion’s Belt” of three stars; the Plough (labelled “Ursa Major” towards the top left, and the sickle of Leo towards the left of the diagram. There is a dashed line labelled “Ecliptic”, and you'll notice that Leo lies roughly on this line. So do Cancer, Gemini, Taurus and Aries. In fact, all twelve of the zodiac constellations lie on or around the ecliptic – and these are the constellations that play the most important part in astrology.

The ancients divided the ecliptic into 12 segments, and identified a key constellation within each segment – for instance, near Orion is a reddish star called Aldebaran, and a few other stars around it. If you imagine that Aldebaran is an “eye”, two of the stars above it look like they could be horns, so it’s possible to “join the dots” and make out a pattern of a bull – that’s the constellation Taurus. Every 30 degree segment of sky has a particular zodiac pattern in it (astrologers work in strict 30 degree segments – some constellations actually sprawl more than this, and others less than this; some even overlap a bit). These 30 degree segments are called signs.

When someone says that they have “Moon in Taurus”, this should mean that at the moment when they were born, if you looked out of the window and saw the Moon, you’d see that it was roughly in the area of the constellation of Taurus. If someone says that they have “Sun in Taurus”, it should mean that if you could somehow blot out the brightness of the sun (like during a total eclipse), you would see that the Sun is roughly in that constellation. This is done for all the planets (Mars, Jupiter and so on). All the planets do lie roughly on the ecliptic – so you never see Mars or the Moon very much higher than the zodiac constellations, nor very much lower.

This idea is then extended, to say “at the moment you were born, what sign of the zodiac was coming up over the eastern horizon?” The answer to that question is your “rising sign” or Ascendant . It’s also why birth times are important in astrology. If you only know your date of birth, you can look at the position of the planets, Sun and Moon at midday, and most of them (maybe not the Moon, as it moves fast) will be correct. However, you can only calculate your Ascendant if you know your time of birth, since during a 24-hour day each sign of the zodiac will “rise” once.

That’s the theory. However, there’s a slight problem. If you looked out of your window on a clear night in early October 2000, you would have seen a bright “star” near Aldebaran, in the constellation of Taurus; that’s actually Jupiter (a planet, not a star). So, if someone was born on, say, 6 October 2000, they must have Jupiter in Taurus, right? Nice theory, but in fact an astrologer would look up that date in an “ephemeris” (a book of positions), or use a computer program, which would tell them that anyone born on that date has Jupiter at 11 degrees of Gemini (in other words, nearly half-way through Gemini). This is because the Earth “wobbles” on its axis once roughly every 26,000 years, so the positions of the constellations shift gradually over the millennia (this behaviour is known as “precession”). If the Earth hadn’t wobbled, Jupiter would not only look as though it’s in Taurus, but the astrological calculations would agree too. However, both astrologers and astronomers use this weird convention where they have an arbitrary starting point (analogous to the Greenwich Meridian Line) called the “Zero point of Aries”, which was at the beginning of Aries around 4000 years ago, but is now almost in Aquarius (hence the song “The Dawning of the Age of Aquarius”). So, in our time it’s not feasible to look up at the sky when someone’s born and say “yes, Jupiter is in Taurus, Cancer is rising...” - we have to use an ephemeris, a computer program or perform some tricky celestial mechanics to find out! (To be fair, the ancient astrologers didn’t simply look at the sky, either, because that technique doesn’t work if it’s daylight, and anyway, not all of the planets are visible at the same time).