Chapter 1

Discovering the Night Sky

Constellations

• A constellation is a section in the sky that contains a group of stars that form a pattern

• The stars even in a constellation are not near each other, they only appear that way from earth

• Stars on average are about 30 trillion miles away from each other

Orion Constellation

Celestial Sphere

• Is an imaginary sphere with the earth at the center. The poles and equator align with those of the earth.

Angle of Right Ascension

• Abbreviated RA, right ascension is a angular distance measured along the celestial equator.

• The units for RA are hours, minuets, and seconds.

• RA goes from 0 to 24 hours in a easterly direction.

• The starting point (0 h) is at the vernal equinox.

Vernal Equinox

• The vernal equinox is a point where the path of the sun crosses the celestial equator.

• The sun is at the vernal equinox at sunrise on the first day of spring.

Angle of Declination

• Abbreviated DEC, declination is measured north or south or the celestial equator.

• The units for DEC are degrees.

• DEC goes from 0 to 90 degrees.

• The starting point (0◦ ) is located at the celestial equator.

Ecliptic

• The ecliptic is the path the Sun appears to take on the celestial sphere.

• The sun takes one year to complete a cycle around the ecliptic.

Earthly Cycles

• The earth spins around an axis that is goes through its center, north, and south pole.

• The earth moves around the sun in an elliptical orbit.

• There is an angle of 23.5 degrees between the earths axis and the plane that it orbits the sun.

Daily Cycle

• The earth takes 24 hours to complete one rotation around its axis.

• It is this motion that causes the day and night cycles.

• The earth is rotating very fast. A point on the earths equator is moving about 1000 miles per hour.

Yearly Cycle

• The earth takes one year to complete its orbit around the sun.

• The earth is traveling about 67,000 miles per hour.

The Seasons

• The seasons occur due to the tilt of the earths rotational axis.

Earth’s axis precession

• The earth’s axis goes through precession.

• This precession is due to the gravitational pull of the sun and moon.

• One cycle of the precession takes 26,000 years.

The Phases of the Moon

• The moon takes about 29.5 days to go around the earth and go through its phases.

Eclipses

• A lunar eclipse occurs when earth’s shadow blocks the sun’s light to the moon.

• A solar eclipse occurs when the moon blocks the sun’s light to the earth.

Scales of the Universe

• In our world there are things that vary greatly in size.

• In order to talk about things that are very small or large we use scientific notation.

• For example a proton is about 10-15 meters while a galaxy is on the order of 1020 meters.