Celestial Coordinates and Observer's Position

Overview

This lecture explains how to represent an observer's position on Earth and in the sky, focusing on celestial coordinates, sky domes, and how latitude affects the position of stars and the Sun.

Representing the Observer's Position

• The point directly above the observer is the zenith; directly below is the nadir.
• Main directions around the horizon are north, south, east, and west.
• The sky dome above the observer is called the observer’s hemisphere.

Locating Celestial Objects

• Stars’ positions are measured using altitude (height above horizon) and azimuth (compass angle from north, measured clockwise).
• Azimuth values: North = 0°, East = 90°, South = 180°, West = 270°.
• Programs like Stellarium can display altitude and azimuth for any star at any location.

The Celestial Equator and Polestar

• The celestial equator is the sky’s projection of Earth’s equator, forming a circle from east to west across the sky.
• The pole star appears above the northern horizon at an angle equal to the observer’s latitude.

Geometry of the Sky at Different Latitudes

• At Tampa (28.1° N), the polestar is 28.1° above the northern horizon.
• The difference between the pole star and the celestial equator is always 90°.
• The zenith is always 90° above the horizon.

Solar Motion and Declination

• The Sun’s maximum declination is +23.5° (summer solstice), minimum is -23.5° (winter solstice), and 0° at equinoxes.
• In Tampa, the Sun can reach a maximum altitude of 85.4° at summer solstice (almost overhead).
• At Anchorage (61.2° N), the polestar is 61.2° above the horizon, and the Sun only reaches 5.3° above the horizon at winter solstice.

Navigation Using the Pole Star

• The altitude of the pole star equals the observer's latitude, making it essential for navigation.
• Navigators used the pole star’s height to determine latitude at sea.

Key Terms & Definitions

• Zenith — Point directly above the observer.
• Nadir — Point directly below the observer.
• Azimuth — Compass angle from north clockwise to locate objects on the horizon.
• Altitude — Height of a celestial object above the horizon.
• Celestial Equator — Projection of Earth’s equator onto the sky.
• Pole Star (Polaris) — Star located nearly above Earth’s North Pole.
• Declination — Celestial object’s angular distance north or south of the celestial equator.
• Observer’s Hemisphere — The half-sky visible from a given location.

Action Items / Next Steps

• Practice identifying zenith, nadir, and main compass points on the sky dome.
• Use Stellarium to find altitude and azimuth of stars for your location.
• Review latitude effects on the sky for different cities (e.g., Tampa vs. Anchorage).