Stars and Constellations Overview

Jul 8, 2025

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.