What is the Celestial Sphere?

• A way of visualizing the sky

• A geocentric (Earth-centered) model Introduction to Astronomy • Stars, planets, and other astronomical objects appear to be positioned on the celestial sphere The Celestial Sphere Presented by Robert Wagner

What is the Celestial Sphere? The Celestial Sphere - Definitions

• Zenith: The point straight overhead for an observer • Horizon: The circle where the celestial sphere meets the Earth • Celestial Pole: The projection of the Earth’s pole to the celestial sphere • Celestial Equator: The projection of the Earth’s equator to the celestial sphere • Ecliptic: The apparent path of the Sun on the celestial sphere • Vernal Equinox: The intersection of the ecliptic and the celestial equator, where the Sun is moving northward http://astro.wsu.edu/worthey/astro/html/im-sky/celestial-sphere-2.jpg

Circumpolar Stars Navigation Through Astronomy

• The altitude of the pole gives the latitude of the observer! • Stars close to the celestial pole will never set! • The pole will be 40 degrees above the horizon for an observer at a • Polaris is near the celestial pole latitude of 40 degrees • Stars revolve in circles around the celestial poles • The pole will be on the horizon for an observer at the equator • Stars at greater distances will cross the horizon • The pole will be at the zenith for an observer at the pole • The north celestial pole will not be visible for an observer south of the equator!

Image Credit: https://commons.wikimedia.org/wiki/File:91_minutes_of_the_night_sky.jpg Motions on the Celestial Sphere Motions of the Celestial Sphere (2)

• Seven known objects that moved among the stars (week) • Daily Motion (Rising and Setting) • The Sun (day and year) • Caused by rotation of the Earth • The Moon (month) • Annual motion • 5 Planets – Mercury, Venus, Mars, Jupiter and Saturn • Caused by the revolution of the Earth around the Sun • Planets • Sun moves about 1 degree/day eastward relative to the stars • Wander among the stars • Slowly travel through the constellations of the zodiac • Retrograde motion • Planets (and other objects near the ecliptic) will also travel through these constellations

Image Credit: http://www2.astro.psu.edu/users/caryl/a10/lec2_2d.html Image Credit: http://hyperphysics.phy-astr.gsu.edu/hbase/Solar/retromars.html

The Constellations Summary

• Ancient Definition: • The celestial sphere is a way of visualizing the sky • Grouping of bright stars • The appearance of the sky will depend on the observer’s location on • Named in honor of mythological figures Earth • Modern Definition: • Planets and stars have different motions on the celestial sphere • Defined as one of the 88 regions of the sky

• Every part of the sky is in one of the 88 constellations • The modern definition of a constellation is different than the ancient definition