Announcements•Homework Set 1 is due today

•Homework set 2: Chapter 2 # 45, 46, 50, 52, 53 & 54

•Use Exam Formula Sheet as you do the homework to familiarize yourself with where things are on it.

Hint for homework problems # 52, 53 & 54

Terrestrial CoordinatesLongitude is measured CCW (+) or CW (-) around from Greenwich England

Latitude is measured North or South of the equator

Both are measured in degrees, minutes and seconds

Celestial CoordinatesThe angle between the celestial equator and the ecliptic is 23.5°

Right Ascension (RA) is measured CCW from the Vernal Equinox and is in hours, minutes and seconds

Declination (Dec) is measured above (+) or below (-) the celestial equator and is in degrees, minutes and seconds

See Appendix A6 for more on celestial coordinates

Finding the CE and NCP at your latitude

Altitude of NCP above due north horizon along the meridian is just f, your latitude (+ for north, - for south)Altitude of the celestial equator above due south horizon along the meridian is 90°-f

ExampleChapter 2 problem # 43: The Moon’s orbit is tilted by about 5° relative to the Earth’s orbit around the Sun. What is the highest altitude in the sky that the Moon can reach, as seen in Philadelphia (latitude 40° North)?

Example SolutionWhat is being asked?...Maximum altitude of the Moon from 40° North latitude.

What information is given?...latitude = 40° N

Tilt angle of Moon from ecliptic = 5°Tilt angle of ecliptic from celestial

equator = 23.5°

Example Solution 2Equation(s) to use: Refer to diagram two slides back. The altitude of the celestial equator above the local horizon is 90° - Latitude

CE = 90° - 40° = 50°

Maximum altitude of Ecliptic = CE + 23.5° = 50° + 23.5° =

73.5°

Maximum altitude of Moon = EclipticMax + 5°

= 73.5° + 5° = 78.5°

Time and AstronomyThe 24 Hour Day?

One rotation of Earth = 1 sidereal day

23 hours 56 minutes 4.091 secondsThis is the time required for the

Earth to complete one rotation with respect to the fixed stars

As the Earth rotates it also moves around

the Sun.

So, for the Sun to return to the same place in the sky the Earth must rotate a little more than one complete rotation

Noon–to–noon isn’t always 24 hours

The Mean Solar Day is exactly 24 hours. It is the time between meridian transits of the Sun averaged over four years

The Year

1 orbit around the Sun = 365.2564 days The sidereal year

1 Tropical Year = 365.2422 mean solar days The time from Vernal equinox to Vernal equinox

Pope Gregory XIII’s CalendarThe Gregorian Calendar (1582)

Most years have 365 days

Years evenly divisible by 4 have 366 days except century years.

Only century years evenly divisible by 400 are leap years

Precession of the Equinox

Like a spinning gyroscope, the Earth precesses. The period of the precession is 25,920 years

The Precession of the Equinox leads to a shift of the celestial

pole

It also shifts the constellations of the zodiac

The Lunar CycleSidereal versus Synodic

The sidereal period is

the orbital period with respect to the “fixed”

stars: 27.32166

days

The Synodic period is the length of the cycle of phases

29.530589 days

Since the angular size of the Sun and Moon are almost the

same, eclipses occur

The Saros Cycle is an eclipse cycle

223 Synodic Months or 18 years 11.3213 days

The Saros Cycle is the repetition cycle of eclipse