Astronomy Picture of the Day

The Ecliptic

• The Ecliptic - the Sun’s apparent annual path among the constellations.

• The Ecliptic Plane - the plane of Earth’s orbit around the Sun.

• The ecliptic and the celestial equator are not the same because the Earth’s rotational axis is tilted with respect to the ecliptic plane.

The Celestial SphereThe ecliptic is the Sun’s apparent path through the celestial sphere.

The Sun moves along the ecliptic a little bit each day (about a degree) as the Earth orbits the Sun.The constellations of the Zodiac

lie along the ecliptic.

Class Action

The sky varies as Earth orbits the Sun

• As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.

• At midnight, the stars on our meridian are opposite the Sun in the sky.

Class Action - Coordinates and Motion - Ecliptic (Zodiac) Simulator

ClassAction - Coordinates and Motion - Seasons Simulator

• Notice the relation of our orbit to the ecliptic.• Note how the tilt of Earth’s rotational axis and

Earth’s position in its orbit about the Sun affect how much direct sunlight shines on the northern and southern hemispheres.

• How is this related to the seasons? • How is this related to the stars that we see each

night?

What causes the seasons?

The tilt of Earth’s axis of rotation produces longer (or shorter) hours of more (or less) direct sunlight

When the Sun is high in the sky during the day, the number of daylight hours is greater and the amount of

direct sunlight received is greater. This results in Summer.

Summer Winter

When the Sun is low in the sky during the day, the number of daylight hours is less and the amount of direct

sunlight received is less. This results in Winter.

Summer Winter

The Real Reason for Seasons• Earth’s axis points in the same direction (to Polaris) all

year round, so its orientation relative to the Sun changes as Earth orbits the Sun.

• Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct.

• AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

• Mars also has seasons because its axis is tilted with respect to the Sun as well.

Why doesn’t distance matter?• Variation of Earth–Sun distance is small — about 3%; this small

variation is overwhelmed by the effects of axis tilt.

How does the orientation of Earth’s axis change with time?

• Although the axis seems fixed on human time scales, it actually precesses over about 28,000 years.— Polaris won’t always be the North Star.— Positions of equinoxes shift around orbit; for example, the

spring equinox, once in Aries, is now in Pisces!

Precession