Star Noon 3 of 3: See Sidereal Time in the Sky

Sidereal Time - a 3-part series

In the previous post, we followed Sirius from its alignment with the Sun in July to its midnight “star noon” on New Year’s Eve — watching the 3-minute-56-second daily drift play out across the seasons. Now, let’s step outside and see sidereal time for ourselves.

See sidereal time in the sky

Look up at the night sky at midnight on a balmy summer evening. If you are lucky, there are a few hundred visible stars in the sky. Depending on the “clock time” and the date on the Earth, a specific group of stars and their constellations is in view. At midnight each day we get a specific window onto the stars - many stars are visible in the sky, but most are below the horizon, behind the Earth. We see stars rising in the east and stars setting in the west.

Take a snapshot of this starry midnight sky and hold it in your mind.

The Earth continually rotates so the stars move in our sky all through the night and continue the next day. At the same “clock time” tomorrow the entire starry half-dome of stars will be visible again, but they will appear to have moved very slightly. Each day, the stars appear to move a little bit toward the west because of the progression of the Earth’s orbit along its “track.” It is this progression that makes the stars seem to rise earlier each day.

Snapshot of the midnight sky

Here’s a good way to think about sidereal time: snapshot the midnight sky every night for a year, then play it back as a time lapse movie. What would you see?

If we “snapshot” the midnight sky (at midnight the Sun crosses the anti-meridian… this happens once every 24 hours) we will see a slow westward movement of stars. The stars move steadily westward because the Earth’s “midnight window” moves eastward (counterclockwise) in orbit.

Our Earth-bound, nighttime window on the sky changes all year long. It’s as if we were in a strange train on a long circular track. As seen from above… day (the Sun) is always visible out the left side, night is visible out the right.

Stars at Opposition

Seen from “above” the solar system, the Sun, the Earth and a star make a straight line; when a star crosses the anti-meridian at midnight that star is at opposition. Stars can cross the anti-meridian at any location from north to south.

Familiar stars that appear along the Zodiac or the celestial equator help us ponder sidereal time.

There is one starry Zodiac region in particular that plays the role of sidereal time “zero”: the spring equinox point which is currently “in” the constellation Pisces. When this point crosses your meridian, sidereal time resets to zero — like noon on a solar clock.

Where sidereal time begins (how to tell time by the stars)

To get to the zero start time in sidereal time we ignore the Sun. We set our clocks to sidereal time zero by the meridian crossing of the stars.

The beginning of sidereal time is the moment the spring equinox sky location (where the Sun crosses upward along the ecliptic past the celestial equator - the so-called “first point of Aries) crosses the observer’s local meridian. The snapshot at this moment is sidereal time “zero.”

You can chase this sidereal time frame over the course of a year (orbit) by changing the time you observe or by changing your location on the surface of Earth.

Telling time by the stars

The circumpolar constellations can tell you the time at night - the solar time. All you need is an ancient device called a Nocturnal, a clear view of the northern sky, and the date.

An illustration showing the Nocturnal, the observer’s eye, and the stars. Johann “nojhan” Dréo, CC BY-SA 3.0, via Wikimedia Commons.

The Nocturnal is an ancient device that helps you to find the solar time from the position of circumpolar stars. But, this is using the stars to tell solar time - it’s fascinating and useful, and it illustrates the point of this article: the stars move in a predictable, yearly motion that can be used to tell time. Sometimes you want to tell solar time but sometimes you want to tell sidereal time.