Overview
This lecture covers how astronomers describe the sky, introduces concepts like the celestial sphere, explains asterisms and constellations, and describes how celestial bodies appear and move in the sky.
Describing the Sky
- Astronomers use the imaginary "celestial sphere" to represent the sky encircling Earth.
- The "celestial dome" is half of the sphere visible from any location.
- The "zenith" is the point directly overhead; the "horizon" is where the dome meets the ground.
- Earth's rotation axis, extended to the celestial sphere, marks the north and south celestial poles.
- The celestial equator is obtained by extending Earth's equator onto the sphere.
- Latitude (north-south angle from equator) and longitude (east-west angle from Greenwich) describe positions on Earth.
Earth's Rotation and the Night Sky
- Earth rotates counterclockwise (from above the north pole) every 24 hours, causing day-night cycles.
- All celestial bodies (Sun, Moon, stars) appear to rise in the east and set in the west.
- At the poles, only half the sky is visible and stars circle the celestial pole without rising or setting.
- At the equator, all stars can be seen throughout a 24-hour period.
Asterisms and Constellations
- Asterisms are recognizable star patterns; examples include the Big Dipper and Orion's Belt.
- Constellations are official regions of the sky (88 total), each containing one or more asterisms.
- Modern constellation boundaries were defined by the International Astronomical Union in 1928.
- The difference: constellations are areas, asterisms are patterns.
- Navigational asterisms like the Big Dipper help find the North Star (Polaris).
The Sky in Three Dimensions
- The stars form three-dimensional structures but are projected as two-dimensional patterns.
- Patterns like Orion’s Belt are apparent only from Earth; stars in them may be very far apart.
- Over thousands of years, star positions and patterns change due to star motion.
- Star clusters like the Pleiades are physically close in space, unlike most asterisms.
The Sun, Ecliptic, and the Zodiac
- The Sun is the closest star to Earth, about 8.3 light minutes away (1 astronomical unit or AU).
- Apparent sky brightness/size of stars depends on their actual distance and luminosity.
- The ecliptic is the plane of Earth’s orbit around the Sun.
- The celestial equator (Earth’s equator projected out) and the ecliptic are tilted by 23.4° relative to each other.
- The intersection points of the ecliptic and celestial equator are the equinoxes.
- The Sun appears to move through 13 constellations (the Zodiac) along the ecliptic during the year.
- Planets, the Sun, and the Moon are always seen within a belt (the Zodiac) about 8-9° from the ecliptic.
Key Terms & Definitions
- Celestial Sphere — Imaginary sphere representing the sky with Earth at its center.
- Zenith — The point directly overhead an observer.
- Horizon — The boundary where the sky meets the ground.
- Celestial Poles — Extensions of Earth's axis intersecting the celestial sphere.
- Celestial Equator — The projection of Earth's equator onto the celestial sphere.
- Asterism — A notable pattern of stars in the sky (not an official region).
- Constellation — An official region of the sky, as defined by the IAU.
- Ecliptic — The plane of Earth's orbit around the Sun.
- Equinox — Points where ecliptic and celestial equator intersect; mark equal day and night.
- Zodiac — The belt around the ecliptic where the Sun, Moon, and planets appear.
- Astronomical Unit (AU) — Average Earth-Sun distance (~150 million km or 8.3 light minutes).
Action Items / Next Steps
- Read Open Stax Astronomy, section 2.1.
- Complete the posted practice questions on Teams.
- (Optional) Download Stellarium or a sky-mapping app to explore the night sky.