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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 6: Daily & Annual Motions

Key Ideas:

Daily Motions Annual Motions

Daily Motions

Objects in the sky appear to rise in the East and set in the West each day. This apparent daily motion is a reflection of the Earth's rotation about its axis.

Apparent Paths

The Apparent Paths of objects are parallel to the Celestial Equator.

Their orientation depends on your latitude:

In Columbus: we are at 40 N, so the paths are tilted by 50 degrees from the horizon.

Circumpolar Stars

Any star closer than your latitude to your visible celestial pole (north or south) will always be above your local horizon.

These are the Circumpolar Stars

The opposite pole's circumpolar stars never rise above your horizon.

Summary of Daily Motions:

Daily Motions of celestial objects reflect the Earth's daily rotation about its axis:

Annual Motion of the Sun

Over the course of a year, the Sun appears to move a little towards the East each day as seen with respect to the background stars. This daily eastward drift is <1° per day (there are 365 days in a year, but only 360° in a circle).

This apparent motion is a reflection of the Earth's annual orbit around the Sun.

The Ecliptic

The Ecliptic is the apparent path of the Sun relative to the stars.

This tilt is called the Obliquity of the Ecliptic

The Obliquity of the Ecliptic varies between 21.2 and 24.5° with a roughly 41000 year period:

The Zodiac

As the Sun moves along the Ecliptic as seen from Earth, it passes through 12 ancient constellations known as the Zodiac.

The Zodical Constellations can be used as a kind of an astronomical calendar:


Solstices occur when the Sun is at its maximum northern and southern declination. The word Solstice is derived from the Latin words "sol sistere" = "sun" and "stand still".

Solstices occur twice a year in June and December:


Equinoxes occur when the Sun crosses the Celestial Equator. Derives from the latin "equinoctis" = "equal night".

Equinoxes occurs twice a year, during March and September.

Length of the Day

The length of the day depends on the location of the Sun along the Ecliptic.

Vernal & Autumnal Equinoxes:

Summer Solstice:

Winter Solstice:

Summary of Annual Motions:

Annual Motions reflect the Earth's orbit around the Sun:
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