Solar vs. Sidereal Day

Solar vs. Sidereal Day

An Earth Day
• Sidereal Day: 23 hr 56 min 4 sec
Motion relative to background stars
• Mean Solar Day: 24 hours
The average time between meridian
crossings of the Sun
• Apparent Solar Day: varies
The 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