Astronomy 103: 2-Sky Coordinates and Constellations

Key Concepts

  • This lecture covers the way we describe and locate objects in the sky, akin to using longitude and latitude on Earth.

Introduction to Sky Coordinates

  • Discussion prompts for students: Introduce yourself and discuss which of several objects is least like the others (options A, B, C, D).

Importance of Patterns in the Sky

  • Patterns in the sky were vital to our ancestors, helping them with agricultural decisions such as when to plant and harvest crops.

  • Example: Petroglyphs showcasing the summer solstice from Peurco Pueblo.

Locating Objects in the Sky

  • Earth Coordinates:

    • Example: Madison's location is specified as:

    • Longitude: 89° 24′ 4″ W

    • Latitude: 43° 4′ 23″ N

    • Geographic regions provide context (e.g., Madison is in Wisconsin).

  • Sky Coordinates:

    • Example of locating a bright star:

    • Sirius, the brightest star in the night sky is located in the constellation Canis Major.

    • Coordinates:

      • Right Ascension (RA): 06h 45m 08s

      • Declination (Dec): -16° 42′ 58″

Understanding Constellations

  • Definition: A constellation is an imaginary image formed by a pattern of stars in the sky.

  • Visible Stars:

    • Approx. 5000 stars are observable to the unaided eye.

    • Each culture has created their own constellations with associated myths.

    • Modern astronomers recognize 88 official constellations, primarily defined by the Greek astronomer Ptolemy in the 2nd century CE.

Astronomical Region Division

  • Astronomers utilize constellation names to indicate specific regions of the sky.

  • Visual representation of the Milky Way as seen from Southern Wisconsin provides an example of how diffuse star light appears across the sky.

The Milky Way Galaxy

  • The diffuse light we observe as the Milky Way originates from numerous faint stars, with darker patches signifying interstellar dust that absorbs starlight.

  • All stars visible to the naked eye are part of the Milky Way Galaxy, highlighting its disk when viewed from Earth.

Structural Features of the Milky Way

  • A map of the Milky Way shows different arms and regions:

    • Scorpio-Centaurus Arm

    • Norma Arm

    • Sagittarius Arm

    • Orion Spur

    • Various galactic longitudes are indicated (e.g., 60° to 300°)

Observations from Earth

  • The Milky Way appears strongest in the Southern Hemisphere, with the 3D center lying near the constellation Sagittarius.

Sky Coordinates Explained

  • Analogous to Earth coordinates, celestial coordinates are given in terms of:

    • Right Ascension (RA)

    • Range: 0 hours to 24 hours (East to West)

    • Declination (Dec)

    • Range: +90° to -90° (North to South)

The Celestial Sphere

  • The celestial sphere model helps in visualizing how stars occupy positions in space, even if they appear close together from Earth's perspective.

  • Example: The stars of the Big Dipper (Ursa Major) are at varying distances in three-dimensional space, although they form a recognizable 2D pattern in the sky.

Determining Star Positions

  • Questions for Students:

    • If you know a star's RA and Dec, what can you determine?

    • Options include the constellation's location or various distances.

Quantitative Analysis of Stars

  • Consider the example of two stars:

    • Star A: RA = 13.0 hr, Dec = +22°

    • Star B: RA = 13.0 hr, Dec = -8°

  • Calculation of sky separation:

    • Separation = | +22° - (-8°) | = 30°

    • Only declination is relevant as both stars have the same RA.

Observational Effects Due to Earth’s Rotation

  • The daily motion of stars can be observed as they rise and set throughout the night due to the Earth's rotation.

  • Earth rotates counter-clockwise from a North Pole perspective.

Local Sky Coordinates

  • Objects are described by altitude (degrees above the horizon) and azimuth (direction along the horizon).

  • The zenith is directly overhead (90° altitude), and the sky is divided into East and West halves by the meridian.

Latitude Effects on Observations

  • The latitudinal position influences which stars rise, set, or remain circumpolar.

    • Example Observations:

    • At the North Pole: Stars near the celestial equator lie on the horizon.

    • At the Equator: All stars rise and set throughout the night.

    • Mid Latitude Regions (e.g., Madison 43° N): Some stars complete full circuits about the pole and are classified as circumpolar, while others rise and set based on proximity to the poles.

Summary of Case Study

  • Despite the rotation of the Earth causing local motion of stars, their positions on the celestial sphere remain fixed. The Sun, however, changes position on the celestial sphere due to Earth's orbit.

Assignments for Upcoming Class

  • Finish reading assigned textbook chapters:

    • Chapter 2 - 2.2: The Reasons for Seasons

    • Chapter 2 - 2.3: The Moon Our Constant Companion

    • Chapter 2 - 2.4: The Ancient Mystery of the Planets

  • Piazza practice assignment due at the end of the day.

  • Homework Quiz #1 due Thursday, with a Math Review Discussion Exercise as well.

Preview Question for Wednesday's Class

  • Understanding full moon visibility:

    • When do we see a full moon?

    • Options include:

      • (A) When the moon is between Earth and the Sun.

      • (B) When opposite from the Sun.

      • (C) Every night between 11:00 PM and 1:00 AM.