What if I was Born in a Leap Year?
The tables of dates used in horoscope columns and mass-market Sun Sign books are based on the average dates the Sun enters each Sign (i.e. when it crosses the Cusp). BUT in any given year the actual date can be a day earlier or later, because our human calendar does not exactly correspond to the actual movement of the Earth around the Sun. While calendars show a year of 365 days, it actually takes 365.25 days for the Earth to return to its original starting point after revolving once around the Sun.
We assign every fourth year as a Leap Year (years divisible by 4) to bring the calendar back into line with reality and adjust for those four quarters of a day by adding one full day on February 29. Being born in a Leap Year does not really affect those "Cusp dates" or the real astrological Cusps in your horoscope chart - but what makes a Leap Year necessary is also WHY the so-called "Cusp dates" are not always the real dates the Sun changes Sign.
Astrological calculations take Leap Years into account when they translate your calendar birth date to "Universal Time", so the Cusps or Signs in your personal horoscope chart are accurate even if you were born in a Leap Year. Even if you were born on February 29th and only have a "real" birthday every four years :-) you can still rely on a birth chart done by a real astrologer to take that into account when interpreting your birth horoscope.
How Signs are Measured
Signs are areas in space which are, in effect, measured by the time it takes for the Earth to revolve around the Sun (the seasonal cycle). But from our viewpoint, the Sun appears to travel across 30 degrees of the zodiac in one-twelfth of a year of 365.25 days. Each of the 12 Signs represents how far the Sun appears to travel in one-twelfth of an Earth year, or one month; and we see the Sun appear to move through one Sign in about 30 days, at the rate of one degree per day.
There are some links below to pages with more details on Signs, but I want to comment here on another way we make adjustments to keep our system of measuring Signs in line with reality.
The majority of astrologers in the Western world (and Australia) use the Tropical Zodiac which measures the Signs from the ever-changing perspective of the planet we live on. The "constellations" along the zodiac, those imaginary patterns among the fixed stars from which the original zodiac "signs" were derived, are NOT in the same position relative to the Earth today as they were 5000 years ago.
The Earth rotates around its axis once per day, but the axis itself very slowly rotates "backwards" in a kind of cone-shaped motion by 30 degrees every 2160 years like a wobbly spinning top. This natural phenomenon is called "precession" and the time of 2160 years it takes the axis to rotate 30 degrees around a circle is called an "Age". Twelve "Ages" comprise one "Great Year" in time.
Since each Sign spans 30 degrees of arc, this motion gradually causes a change in the Sign which appears to rise at dawn each year on the day of the Vernal Equinox, the first day Spring (around March 19-21 in the Northern Hemisphere). Dawn is the moment when darkness first turns into daylight, when the Sun first appears above the eastern horizon.
Think of it this way... we are living on a spinning "top" (the Earth) which moves us around and around as it spins on its axis. If it remained exactly vertical, we would see the same fixed star right above us every 24th hour as the top started its next daily rotation. But if the top was "tipped" slightly away from the vertical, and its axis was rotating very slowly, then each time we looked right above us at the same time each night that star we saw last night would appear to have moved just a little. In fact, all the stars would have shifted a bit, and over the course of many years, we would actually notice that a different star is right above us. This is why the "North Star" which is directly above the Earth's northern axis eventually appears to move away from its alignment with the axis (the true "north") and another "North Star" takes its place.
If you set a time-lapse motion picture camera on a tripod and pointed it directly towards the North Star, it would record a series of concentric circles of light which represent the stars moving across the night sky. At the center of the circles would be the North Star, the only star which does not appear to move over the course of a night, because it is on the same axis the Earth spins around. But as the Earth's axis precesses over the years, the current "North Star" will no longer be exactly in line with it - and that star too will create a very small circle when viewed with time-lapse photography.
Depending upon how far north or south of the Equator we live, we will see different stars directly above us, but ALL the stars will appear to shift over the course of many years. And so will the star patterns we call "constellations". They will still be there, but will not appear in exactly the same direction as related to the Earth's axis and to us.
So when the Sun appears on that first day of Spring next year at a slightly different point in the constellation of Pisces, we Tropical astrologers will re-define that new point as "0 degrees of the Sign of Aries". It will be only a fraction of a degree slightly farther back in the constellation of Pisces than last year; but over centuries in time it will have moved back from the end of Pisces to the beginning of Pisces and touch the end of the constellation of Aquarius - which will officially mark the beginning of the Age of Aquarius.
We "tropicalists" do not stay stuck in a 5000 year old system of astrology (known as "Sidereal" astrology - "of the stars"). We allow our Astrology to constantly evolve to represent the reality of living on a planet that slowly shifts its alignment with the stars - now that we understand the phenomena of "precession of the equinoxes". Tropical Astrology is "earth-centered" astrology. So remember that the Sun appearing in the sky in a certain constellation does NOT mean that it is in the Sign of the same name - if you are using Tropical Astrology instead of Sidereal Astrology.
Actually, you can't see the constellations when the bright Sun is in the sky, but all those stars really are there, and the Sun is passing through some set of stars along that particular band of the sky we call "the zodiac". (The Sun and Moon and planets never appear higher or lower in the sky than within that narrow band.) At night, we may be able to see the Moon and up to five planets within some of those constellations which are visible along the zodiac (the other planets are too far away to be visible).
Just remember that in the Tropical System the Sign of the same name as a constellation is actually located one whole constellation ahead from where that constellation appears in the sky. If the ancient constellation of Pisces is rising on the horizon, it is actually today's tropical Sign of Aries that is rising. The sidereal Sign of Aries is still the same as the constellation of Aries; since the Sidereal System is concerned only with the movement of planets (like Earth) through the original constellations, not with how those constellations are changing their alignment to the Earth's axis and equator. Remember: Tropical is Earth-centered, and Sidereal is Star-centered. They use different reference points.
The constellation of Pisces now represents the tropical zodiac Sign of Aries, since we are still in the "Age of Pisces". Confused? Don't worry about it. Your astrologer will get the Signs right using his computer. We don't need to look at the sky any more to cast a horoscope chart.
Astronomers still use the Sidereal System's original constellations and their mythological names. This makes sense, because scientists are not concerned with astrological Signs or "interpreting" the significance of planetary positions and how they relate to the human experience. They just use the fixed stars and constellations as stable reference points to observe and describe where the moving planets and other celestial objects appear in the sky.
When we reach the Age of Aquarius in a hundred years or so, it will be the very end point of the constellation of Aquarius that rises on the eastern horizon at dawn on the day of the Vernal Equinox. We Tropical astrologers will be calling that point "0 degrees of Aries" because we always define that rising point as 0 degrees of the first Sign, which is called "Aries". Our Tropical system of astrology, which uses the Tropic of Cancer and the Tropic of Capricorn on Earth to define the locations of the zodiac Signs, is thus earth-centered, and not star-centered, which makes sense because we are earthlings.
Let's face it, all those constellations are imaginary constructs, not real objects. The star patterns just seemed to look like certain mythologial figures or animals to some ancient astronomers. But the changing alignment of the Earth to the Sun and stars is real, and we measure the Signs now with astronomical calculations done by computers to keep our zodiac signs in line with reality.
Since the zodiac Signs are our reference points for describing where the Sun and Moon and planets and the House Cusps are located in a horoscope chart, we need to make sure our reference points are accurate reflections of our reality. As the Earth is slowly but constantly changing its alignment with the fixed stars, we constantly change our reference points to align our system of Astrology with the planet we live on, and with the celestial bodies which form part of that system. We Tropical Astrologers tend to think it is more important to align our system of Astrology to our own time, and to our own planet Earth, and to our own selves; than to maintain an ancient system whose original alignment was based on the star patterns of 5000 years ago (whose inventors probably did not know about the slow but constant shifting of the Earth's axis).
An ancient astrological saying tells us, "As above, so below" - and to keep that relationship relevant, we keep changing our reference points in the same way the Earth keeps changing its relationship to the stars above.
When you know someone's "Sign" (which really means their "Sun Sign" - the Sign the Sun was in at their time of birth) you can tell that they were born on one of 30 (or 31) days which fall in the range of dates given in the typical Sun Sign Table. But remember that the dates are only "typical" or "average" dates the Sun moves into the next Sign; so if they were born ON one of those dates, or even on the day before or after it, their Sun might actually be in the adjacent Sign, and NOT in the one listed in that table of dates. The only way to know for sure is to get a real birth chart from someone who knows Astrology, and give them your hour and minute and place of birth (the town and country) - not just the month, day and year.