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 Sphere The 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 illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful tool for learning about the sky. When discussing this slide, be sure to explain: North celestial pole South celestial pole Celestial equator Ecliptic It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model. The constellations of the Zodiac lie along the ecliptic. 3

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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. Use this interactive figure to explain how the constellations change with the time of year. Class Action - Coordinates and Motion - Ecliptic (Zodiac) Simulator 6

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? Misconceptions about the cause of the seasons are so common that you may wish to go over the idea in more than one way. We therefore include several slides on this topic. This slide uses the interactive version of the figure that appears in the book; the following slides use frames from the Seasons tutorial on the Astronomy Place web site. The tilt of Earth’s axis of rotation produces longer (or shorter) hours of more (or less) direct sunlight 8

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. 11

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 can be demonstrated in class in a variety of ways. E.g., bring a top or gyroscope to class, or do the standard physics demonstration with a bicycle wheel and rotating platform. You may wish to go further with precession of the equinoxes, as in the Common Misconceptions box on “Sun Signs” --- this always surprises students, and helps them begin to see why astrology is questionable (to say the least!). Can also mention how Tropics of Cancer/Capricorn got their names from constellations of the solstices, even though the summer/winter solstices are now in Gemini/Sagittarius. Precession 12

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