An Earth Day

1
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 of the Sun

2
Solar vs. Sidereal Day
3
Lunar Month

• Synodic month 29.53 days. Defined by the period
  of the phases.
• Sidereal month 27.32 days. Defined by the
  motion relative to the stars.
• Anomalistic month 27.55 days. Defined as time
  between perigees.

4
Synodic vs. Sidereal Month
5
An Earth Year

• Julian Year 365.25 mean solar days. Used in
  calendars before 1582.
• Gregorian Year 365.2425 mean solar days. Used
  in calendars today.
• Sidereal Year 365.2564 mean solar days. Defined
  by the motion relative to stars.
• Tropical year 365.2422 mean solar days. Defined
  from equinox to equinox. Thus, related to the
  seasons.

6
Earth Orbits Sun
7
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 hour circle that passes through the
  zenith and both celestial poles

8
Directions on the Local Sky

• 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
  objects hour circle (west is positive)

9
The Celestial Sphere
10
Celestial Coordinates

• 1 hour 15 degrees
• 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 objects
  hour circle with the equator

11
Measuring Time

• Apparent Solar Time The actual hour angle of
  the Sun 12 hours (using 24-hour clock).
• Mean Solar Time The hour angle of the average
  Sun 12 hours
• Universal Time Mean Solar Time at Greenwich,
  England
• Standard Time Time assigned by time zone

12
Time Zones
13
Measuring Time

• Equation of Time the difference between the
  apparent solar time and mean solar time
• Local Sidereal Time Time measured by the
  position of the stars.
• Local Sidereal Time Right Ascension of objects
  at the Merdian
• Local Sidereal Time Right Ascension Hour Angle