Overview
This lecture explains the difference between circumpolar and seasonal stars, showing how Earth's rotation and orbit affect which constellations we see throughout the year.
Circumpolar Stars and Constellations
- Circumpolar stars are visible all year from a specific latitude because they never set below the horizon.
- The Big Dipper (part of Ursa Major), Ursa Minor (the Little Dipper), and Cassiopeia are circumpolar constellations from the northern hemisphere.
- Circumpolar constellations appear to rotate around Polaris, the North Star, due to Earth's daily rotation.
- As you move farther from Polaris, stars make larger circles and are not always visible all night.
Seasonal Stars and Constellations
- Seasonal stars or constellations are only visible during certain seasons.
- As Earth orbits the Sun, our night view changes, revealing different constellations at different times of year.
- Winter constellations like Orion and Gemini are visible at night in winter but not in summer due to the Sun’s glare.
- Summer constellations such as Scorpius and the Summer Triangle are visible at night in summer.
The Summer Triangle and Scorpius
- The Summer Triangle consists of the stars Vega (in Lyra), Altair (in Aquila), and Deneb (in Cygnus).
- Scorpius is a prominent summer constellation in the southern sky, shaped like a scorpion.
- The brightest star in Scorpius is Antares, which is red and called the “rival of Mars” for its color.
Observing Tips
- Circumpolar constellations can be seen every night in the northern sky.
- Seasonal constellations require observing during the appropriate season.
- Watching the night sky over several hours or days reveals the rotation and movement of the constellations.
Key Terms & Definitions
- Circumpolar stars — stars that never set and are visible all year from a given latitude.
- Seasonal stars — stars visible only during certain seasons due to Earth’s orbit.
- Polaris (North Star) — the star nearly aligned with Earth’s north rotational pole.
- Summer Triangle — a prominent summer asterism formed by Vega, Altair, and Deneb.
- Constellation — a recognized pattern of stars forming an image or shape.
Action Items / Next Steps
- Go outside at night to identify circumpolar and seasonal constellations.
- Observe how different constellations move and change with the seasons.