Solar vs. Sidereal Day

An Earth Day

• Sidereal Day: 23 hr 56 min 4 secMotion relative to background stars

• Mean Solar Day: 24 hoursThe average time between meridian crossings of the Sun

• Apparent Solar Day: variesThe actual time between the meridian crossings

Earth Orbits Sun

The Ecliptic

Tilt of Spin Axis

Ecliptic: Path of the Sun

The Equinoxes and Solstices

• Tilt of the spin axis is related to the seasons

• Effect of tilt on climate is not instantaneous

Ecliptic on the Celestial Sphere

An Earth Year

• Based on the motion of the Sun on the sky or the motion of Earth around the Sun

• Julian Year: 365.25 mean solar days. Used in calendars before 1582.

• Gregorian Year: 365.2425 mean solar days. Used in calendars today.

An Earth Year

• Sidereal Year: 365.256363 mean solar days. Defined by the motion relative to stars.

• Anomalistic Year: 365.259635 mean solar days. Defined as perihelion to perihelion.

• Tropical year: 365.242190 mean solar days. Defined from equinox to equinox. Thus, related to the seasons.

Sun Changes Declination

The Arctic and Tropical Regions

• How are the arctic and tropical regions defined?

• Can you answer this without looking at your books?

• Have you ever been to these regions?

Back to the North Pole

Cold Arctic Regions

Land of the Midnight Sun

Back to the Equator

Tropical Heat

• In the Tropics, Sun is always close to zenith

• In the Tropics, the sun reaches the Zenith at some point in the year

• Lots of heating due to sunlight