Geology/Physics 360

AstronomyChapter 1: The Celestial Sphere

The celestial sphere and the celestial equator

• The celestial sphere is what is above you when you walk outside

• It acts as a model of the heavens• Where the sky meets the ground is the

horizon• Two points in the sky do not move as the

earth rotates (the north and south celestial poles)

• Polaris the north star is at the celestial north pole• The earth rotates counterclockwise (as viewed from

above) therefore stars rise in the east and set in the west (so does the sun and the moon)

• The line that the sun traces across the celestial sphere is the ecliptic

• The sun reaches 23.5 degrees N of the celestial equator on June 21 (the solstice)

• When does it reach 23.5 degrees S of the celestial equator?

• When does it reach the celestial equator? (the equinox)

How would stars rotate if you were at the North Pole?

As the earth moves around the sun the constellations in the sky tend to shift also

Revolving = motion around the sun

Rotation = the spinning motion what causes night and day

Stonehenge in England probably was built to help people understand the seasons

The simplest way to determine a star’s position on the celestial sphere using the topo-centric or the altitude-azimuth coordinate system

The equatorial coordinate systemThe declination is measured in degrees above the celestial equatorThe right ascension is measured in hours, minutes and seconds in the easterly direction from the vernal equinox position on the celestial equator

Screen capture of HNSKY with cursor positioned on Arcturus

We will briefly discuss the orbit of the moon in class. We will return to the moon later in the semester (it has its own chapter)

The moon goes through its cycle of phases in 29.5 days however the sidereal month is the time the moon takes to complete an orbit relative to the distant stars of one complete cycle. This we bring into play a new time system and a new concept of measuring time. Sidereal time or time according to the stars.

The orbit plane of the moon around the earth is slightly tilted with respect to the ecliptic. This is what causes solar and lunar eclipses.