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

Synodic vs. Sidereal Month

Lunar Month

• Sidereal month: 27.321662 days. Defined relative to the stars.

• Tropical month: 27.321582 days. Defined relative to the celestial equator.

• Anomalistic month: 27.554550 days. Defined as perigee to perigee.

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.

Definitions Zenith: The point on the sky that is directly

overhead of the observer. Horizon: The great circle on the celestial

sphere that is 90 degrees from the zenith Hour circle: The great circle through the

position of a celestial body and the celestial poles

Meridian: The great circle passes through the zenith and both celestial poles

Even more definitions

Altitude: The minimum angular distance between the position of a celestial body and the horizon

Azimuth: The angular bearing of an object, measured from North (0 degrees) through East (90 degrees), South (180 degrees), West (270 degrees), and back to North (360 degrees)

Hour angle: The angle between the meridian and an object’s hour circle (west is positive)

Even more definitions

• Declination: The minimum angular distance from the position of a celestial body and the celestial equator

• Right Ascension: The eastward angle from the vernal equinox to the intersection of an object’s hour circle with the equator

• An hour of angle = 15 degrees

Motion on the Sky

Time Zones

Motion Depends on Declination

The Sky at the North Pole

• At the North Pole, the North Celestial Pole is at the zenith

• Stars never rise or set• Planets, Moon, and

Sun do rise and set…Why?

Stars Rise and Set at the Equator

The Sky at Our Latitude

The Sky at Southern Latitudes

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

Back to the Equator

Cold Arctic Regions

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

Land of the Midnight Sun

The Sky for Navigation