Constellations: Visual groupings of stars (Fig. 2-5). Fig. 2-5 (a) The constellation of Orion. The brightest stars are connected to form the outline of a hunter. (b) An actual photograph of this region. Modern definition: Regions on the celestial sphere with well defined boundaries. Totally 88 today, some added in modern days (e.g., Telescopium). Usually no real correlation among the stars in the same constellations; they could be very far away from each other (Fig. 2-6). They rise and fall with the rotation of the celestial sphere. Which ones we can see depend on the observer's location on Earth (Fig. 2-1). e.g. An observer living in the northern hemisphere on Earth can never see some southern constellations (they never rise above the horizon), but he/she can always see some constellations around the N Celestial Pole (they never set). Fig. 2-6 The true space locations of the stars in Orion, in 3-dimensions. Stars in the constellation are actually very far away from each other. They just appear to be in about the same direction as seen from the earth. Apparent motions of the sun: It moves across the sky daily (the earth rotates on its axis). the celestial sphere rotates with the sun and the stars once a day. It drifts eastwards slowly throughout a year (the earth revolves around the sun). the sun moves across the background of stars on the celestial sphere, completing one cycle every year. Ecliptic: The path of the yearly motion of the sun on the celestial sphere (Fig. 2-7). The ecliptic passes through 13 constellations (12 constellations in astrology). Fig. 2-7 As the earth moves in its orbit, we see the sun in front of different constellations. In January the sun is in front of Sagittarius, but by March it has moved along the ecliptic to Aquarius. The ecliptic is the projection of the earth's orbit onto the celestial sphere. At night, we can only see constellations at the opposite side of the sun e.g.,  In January, we can see Gemini but not Sagittarius (Fig. 2-7). Ecliptic intersects the celestial equator at 23.5° .