The sky — astronomy

Your Birth Time: How the Sky Changes in Minutes

Because Earth rotates, the sky over any place turns about 1° every 4 minutes, and within roughly two hours a very different part of the ecliptic is rising on the horizon. Your birth time decides which stars and constellations were in the east, south and west. That is why sky calculations need the time, the place and the time zone.

Earth's Rotation and the Rising Constellations

When we watch the night sky, it seems that the stars are moving. In reality it is Earth that turns. One full rotation relative to the distant stars (a sidereal day) takes about 23 hours and 56 minutes. The result is the apparent daily motion: stars, the Sun, the Moon and the planets rise in the east, climb, cross the meridian, and set in the west.

The celestial sphere appears to complete a 360° circle, which works out to about 15° per hour, or roughly 1° every 4 minutes. So two views of the sky from the same spot a few minutes apart look almost identical, while views a few hours apart look completely different.

Particularly important is the ecliptic, the Sun's apparent yearly path, along which the Moon and planets also travel. It crosses the horizon at an angle that depends on the time, the date and the latitude. So the point where the ecliptic rises over the eastern horizon moves quickly: on average the full 360° circle passes the eastern horizon in one day, about 30° every two hours. The real rate is not uniform and depends on latitude and on which part of the ecliptic is rising.

What Changes Fast and What Changes Slowly

Not everything in the sky changes at the same pace. It helps to think in three levels.

Very fast: the view relative to the horizon. The altitude and direction of every celestial object change minute by minute. As Earth rotates, the constellation rising on the eastern horizon keeps changing. This is why the rising point of the ecliptic (in the astrological tradition it is called the ascendant) is the most sensitive to time. The astrological division of the sky into sectors called houses changes along with it.

Gradually: the Moon. The Moon orbits Earth in about 27 days and moves against the stars by roughly 13° a day, or about half a degree per hour, which is about its apparent width. Over one day its position among the stars changes noticeably, while its phase changes only slightly.

Slowly: the Sun and planets. The Sun shifts against the stars by about 1° a day. Mercury and Venus move faster than the outer planets, but over a few hours their positions against the stars barely change. Jupiter, Saturn and the more distant planets creep along even more slowly. So two births a few hours apart usually share nearly the same planetary positions but a very different view of the sky relative to the horizon.

The takeaway is simple: the date largely decides where the planets are against the stars, while the time and place decide how the sky is turned above the horizon. For more on timekeeping by the stars, see sidereal time.

Where to Find Your Exact Birth Time

The most reliable source is an official record. Options include:

• A birth certificate. In many countries the time is written on the original or on the full-length copy.
• Hospital records. Discharge papers or the maternity ward file often include a time.
• Relatives. Parents or family members sometimes remember the time or noted it in a baby book, a diary or a card.
• Civil registry offices. You can usually request a copy of the original entry.

Memory alone is unreliable: people often round to something like “around noon” or “early in the morning”. If you only have an approximate time, record it as approximate. What to do then is covered in What if I don't know my birth time. For how we treat personal data, see Personal and birth data.

Local Time, Time Zone and UTC

The time on the record is local time, as shown on clocks at the birth place. To calculate the sky precisely it has to be converted to a single scale. That scale is UTC (Coordinated Universal Time), the world standard by which clocks everywhere are regulated.

The conversion needs three things:

1. The place, meaning city and country, which give the coordinates and the time zone.
2. The time zone, meaning the offset from UTC at the moment of birth. More in the article on time zones.
3. Daylight saving time. In many countries clocks move by an hour in summer. The rules have changed over the decades and differ between countries.

For historical changes, the time zone database (the tz database, also called the IANA time zone database) is used. It collects the rules of various regions over the years. The same calendar date in different years can carry a different UTC offset, so it is not safe to assume that a place always had the offset it has today.

If a time was recorded before standard time was introduced, local solar time may have been used, which depends on longitude. Such cases deserve extra care.

Finally, exact positions of celestial bodies are computed from ephemerides, tables of positions over time (see Ephemerides and accuracy). With a correct UTC time and the right place, the calculation reproduces the sky as it stood over that place at that moment. In the astrological tradition this view is used to draw a natal chart, but the astronomy of the turning sky stands on its own, independent of any interpretation.

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