Topic 2.8: Telling Time by the Sky

For thousands of years the sky was the only clock and calendar there was. You can still use it, and with a stick and a pencil you can do some of what the ancient astronomers did.

Shadows Tell Time

The Sun is highest in the sky at solar noon, and then it casts its shortest shadow, pointing due north (in the Northern Hemisphere). Through the day the shadow swings from west to east.

A sundial

A sundial has a pointer, the gnomon, that casts a shadow on a marked dial. To work well, the gnomon is tilted so it points at the north celestial pole. That means the angle between the gnomon and the ground equals the latitude of the place. In Montgomery that angle is about 32 degrees.

Do-It-Yourself: The Shadow Stick

1. Push a straight stick (about 1 meter long) straight up into level ground in a sunny spot.
2. Mark the end of the shadow with a stone every 15 minutes through the late morning and afternoon.
3. Join the marks with a curved line. The shortest shadow marks solar noon, and a line from the stick's base to that point points north-south.
4. A line across the first and last marks points east-west.

(Eratosthenes used shadows from a pole in this way to measure the Earth. You can try the same trick in Topic 3.1.)

Why Solar Noon Isn't Always 12:00

• Time zones. All the clocks in a time zone share the same time, but the Sun reaches its highest point earlier at the eastern edge of a zone than the western edge. Alabama is about 300 km wide, which is more than 3 degrees of longitude, and the Sun's position shifts by 4 minutes for each degree. So solar noon in western Alabama comes about 13 minutes after solar noon in eastern Alabama, though every clock in the state shows the same time.
• Daylight saving time moves the clocks one hour ahead for part of the year.
• The equation of time. Because Earth's orbit is slightly oval and its axis is tilted, solar noon shifts by up to about 16 minutes earlier or later through the year. If you photograph the Sun at the same time each day for a year, it traces a figure-8 called an analemma.

Other Ways the Sky Tells Time

• Stars as a clock: the sky turns 15 degrees per hour. Sailors used the Big Dipper's position around Polaris to estimate the hour at night.
• The Moon gave the month. Many calendars are built on it.
• Sunrise points swing along the horizon through the year, which tells the season. Stonehenge and many other ancient monuments line up with these.

From Sky to Calendar

Our modern calendar is the Gregorian calendar, adopted in 1582 by Pope Gregory XIII. It keeps the year at an average of 365.2425 days with one rule: every year divisible by 4 is a leap year, except century years, which must be divisible by 400. So 2000 was a leap year, but 1900 was not.

Worth a Pause

Ecclesiastes famously says, "To every thing there is a season, and a time to every purpose under the heaven" (Ecclesiastes 3:1). Before clocks, the sky was how people knew which time it was. A good teacher of astronomy ought to be a small kind of time-keeper too, helping a student learn to look at the Sun's shadow, and know, without a phone, when it is noon.

Practice Problems

1. What is the gnomon of a sundial, and how should it be tilted?
2. When is a shadow shortest, and in which direction does it point in Alabama?
3. Describe how to find north using a stick and shadows.
4. Give two reasons why solar noon is not always exactly 12:00 on your watch.
5. What is the analemma?
6. Is 1900 a leap year in the Gregorian calendar? Is 2000?
7. In which direction does the Sun's shadow move during the day?