Ch. 2

Patterns in the Night Sky

  • From Earth, the universe looks like a celestial sphere, which is an imaginary sphere surrounding Earth on which all celestial objects appear to lie.

  • Constellations are regions of the sky with well-defined borders. Astronomers use the familiar patterns of stars within constellations to locate these regions.

  • The Milky Way, a band of light that circles the celestial sphere, is our view into the disk of our galaxy.

  • An observer's local sky is the dome of the sky they see from their location. The horizon is the boundary between Earth and the sky, the zenith is the point directly overhead, and the meridian runs from due south to due north through the zenith.

  • Altitude and direction are used to pinpoint the position of an object in the local sky.

  • The angular size of an object is the angle it appears to span in your field of view.

  • The angular distance between objects in the sky is the angle that appears to separate them.

  • Stars rise and set because of Earth's rotation.

  • The constellations visible depend on an observer's latitude because latitude affects the locations of the horizon and zenith relative to the celestial sphere. The altitude of the celestial pole in your sky is equal to your latitude.

  • The constellations visible change throughout the year due to Earth's orbit around the Sun.

The Reasons for Earth's Annual Seasons

  • The tilt of Earth's axis, which remains pointed in the same direction in space throughout the year, causes sunlight to fall differently on Earth at different times of the year. This results in the seasons.

  • The June solstice occurs when the Northern Hemisphere is tipped most directly toward the Sun and receives the most direct sunlight.

  • The December solstice occurs when the Northern Hemisphere receives the least direct sunlight.

  • The March equinox occurs when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly toward the Sun.

  • The September equinox occurs when the Northern Hemisphere first starts to be tipped away from the Sun.

  • The Sun's path across the sky changes on the solstices and equinoxes. The Sun rises precisely due east and sets precisely due west only on the equinoxes.

  • Earth's axis also undergoes precession, a slow wobble that takes about 26,000 years. Precession changes the points in Earth's orbit at which the solstices and equinoxes occur and therefore changes the constellations that we see at those times.

Summary

The sources explain the apparent motion of celestial objects in the sky, including the daily rising and setting of stars and the changing positions of the planets, Sun, and Moon. The sources also discuss how Earth's tilt and orbit around the Sun cause the seasons.

Key Terms and Definitions

  • Celestial sphere: An imaginary sphere surrounding Earth on which all celestial objects appear to lie.

  • Constellations: Regions of the sky with well-defined borders.

  • Milky Way: The band of light we see in the sky that traces the disk of our galaxy.

  • Local sky: The dome of the sky we see from our location.

  • Horizon: The boundary between Earth and the sky.

  • Zenith: The point directly overhead.

  • Meridian: An imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.

  • Altitude: The angular distance between an object and the horizon.

  • Angular size: The angle an object appears to span in your field of view.

  • Angular distance: The angle that appears to separate a pair of objects in the sky.

  • Circumpolar stars: Stars that never set below the horizon.

  • Latitude: A measure of north-south position on Earth.

  • Longitude: A measure of east-west position on Earth.

  • Axis tilt: The angle between a planet's rotation axis and a line perpendicular to its orbital plane.

  • Solstice: An event that occurs when the Sun reaches its most extreme position north or south of the celestial equator.

  • Equinox: An event that occurs when the Sun is directly over Earth's equator.

  • Precession: The slow wobble of Earth's axis.

Equations

Angular size, physical size, and distance: For objects that are far enough away (angular size is less than a few degrees), you can relate the object’s angular size in degrees to its physical size and distance from you with the following formula:

Angular size = (physical size / distance) x (360° / 2π)

Methods

The sources use the following methods to explain the phenomena discussed:

  • Observation: The sources describe the patterns of motion and other phenomena observed in the night sky.

  • Modeling: The sources use models, such as the celestial sphere, to represent and explain the observed phenomena.

  • Geometry: The sources use geometrical reasoning to explain phenomena such as the seasons and eclipses.