EARTH IN SPACE

A reminder about earth  The earth is almost a sphere  We locate points on the sphere with 3 coordinates – Longitude (180º W º E) – Latitude (90º S -- 90º N) – Altitude (m above/below sea level)  These are referred to as Terrestrial Coordinates

Finding Polaris: The North Star  Polaris is nature’s compass  Draw a line straight from Polaris to the horizon with your finger and you will be pointing toward geographic north

Position of Polaris  Polaris stays in the same spot throughout the night.  We say it has the same altitude and azimuth throughout the night.  Altitude and azimuth are the local coordinates

Celestial Coordinates  Declination and right ascension:.  Declination is the same as latitude  Right ascension is the same as longitude.

CELESTIAL MOTION  Every celestial object changes position over time (it is in motion).  This motion is not random.  The movement is from East to West.  All move at the same rate.  Celestial objects appear to follow a circular path at a constant rate of 15 degrees per hour (or one complete circle every day).  This motion is called the apparent daily motion

Celestial Sphere

CELESTIAL SPHERE  An imaginary sphere surrounding the Earth.   on which all celestial objects would appear.   Celestial object = any object in the sky that appears to rise in the east and set in the west. Examples: moon, sun, stars, planets.  Celestial poles are directly over Earth’s poles  Celestial equator lies over Earth’s equator  Zenith is a point directly above the observer  Celestial meridian is the circle that runs through the celestial poles and the zenith.  As Earth rotates from west to east, all objects in the sky appear to move from east to west, revolving around the north celestial pole

What About Earth’s Motions? Two Motions: Rotation: Spinning of Earth on it’s axis Revolution: Motion (orbit) or Earth around the Sun. How do we know Earth rotates?  Day/Night  Coriolis Effect  Foucault Pendulum

Rate of Rotation  How many degrees are in a circle? 360  How many hours are in a day? 24  If you divide the number of degrees by the number of hours, you get the degrees the Earth rotates per hour!  The Earth rotates 15°/hour

Path of a Star   Stars that are overhead: follow the same path as the sun (rise in the east and set in the west).   Stars that are close to Polaris: (Celestial North Pole) – appear to move in circles around Polaris (circumpolar stars) – They are in the Northern sky.   Stars that are in the Southern Sky: Rise and set near the horizon because they are mainly visible in the Southern hemisphere.

Earth Rotation = Sky Rotation Polaris - The North Star Circumpolar Star *image taken from Noncircumpolar Star Horizon - where the sky meets the earth

Constellations  Apparent pattern that people use to mark the position of the stars  88 Constellations  The sky is divided into these 88 regions, each associated with a constellation  Constellations change with the season