Astronomy Picture of the Day

1
Astronomy Picture of the Day
2
The Ecliptic

• The Ecliptic - the Suns apparent annual path
  among the constellations.
• The Ecliptic Plane - the plane of Earths orbit
  around the Sun.
• The ecliptic and the celestial equator are not
  the same because the Earths rotational axis is
  tilted with respect to the ecliptic plane.

3
The Celestial Sphere
The ecliptic is the Suns 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.
4
(No Transcript)
5
Class Action
6
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
7
ClassAction - Coordinates and Motion - Seasons
Simulator

• Notice the relation of our orbit to the ecliptic.
• Note how the tilt of Earths rotational axis and
  Earths 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?

8
What causes the seasons?
The tilt of Earths axis of rotation produces
longer (or shorter) hours of more (or less)
direct sunlight
9
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
10
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
11
The Real Reason for Seasons

• Earths 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 doesnt distance matter?

• Variation of EarthSun distance is small about
  3 this small variation is overwhelmed by the
  effects of axis tilt.

12
How does the orientation of Earths axis change
with time?

• Although the axis seems fixed on human time
  scales, it actually precesses over about 28,000
  years.
• Polaris wont always be the North Star.
• Positions of equinoxes shift around orbit for
  example, the spring equinox, once in Aries, is
  now in Pisces!

Precession
13
(No Transcript)
14
(No Transcript)