HC

ASTR 1P01 Lecture 2 Summary

Understanding the Sky

  • Astronomers describe the sky using an imaginary model known as the celestial sphere. This conceptual tool, dating back to ancient Greek astronomy (e.g., Ptolemy), allows us to project celestial objects (stars, planets, etc.) onto a perfectly spherical, infinitely large surface centered on Earth.

  • The celestial sphere is not a physical object but a highly effective visual aid for understanding, mapping, and predicting the positions and apparent motions of celestial bodies from an Earth-bound perspective.

Celestial Concepts

  • Celestial Dome: For any observer on Earth, exactly half of the celestial sphere is always visible above their local horizon. This dome is precisely defined by two key points and a plane:

    • The zenith is the point on the celestial sphere directly overhead an observer.

    • The nadir is the point directly opposite the zenith, below the observer's feet.

    • The horizon is the imaginary plane tangent to Earth at the observer's location, extending outwards to the celestial sphere, where the dome of the sky appears to meet the ground.

  • Celestial Poles: The extended rotation axis of Earth intersects the celestial sphere at two fixed points: the North Celestial Pole (NCP) and the South Celestial Pole (SCP).

    • These poles serve as the immovable pivots around which all other celestial objects appear to rotate over a 24-hour cycle.

    • Polaris, commonly known as the North Star, is located very close to the North Celestial Pole, making it an invaluable navigational aid in the Northern Hemisphere as it appears almost stationary in the sky.

  • Earth's Rotation: A full rotation of Earth on its axis occurs approximately every 24 hours (a solar day). This eastward rotation is the primary reason for the apparent daily motion of all celestial bodies (Sun, Moon, stars) from east to west across the sky, causing them to rise in the east and set in the west.

    • It's important to distinguish between a sidereal day (Earth's rotation relative to distant stars, roughly 23 hours and 56 minutes) and a solar day (Earth's rotation relative to the Sun, approximately 24 hours). The slight difference accounts for the gradual shift in which stars are visible at the same time each night.

Celestial Navigation

  • Latitude & Longitude: These terrestrial coordinates define a specific location on Earth:

    • Latitude captures position north/south of the equator (0 ext{°} at the equator, 90 ext{°} ext{N} at the North Pole, 90 ext{°} ext{S} at the South Pole).

    • Longitude measures position east/west of the prime meridian (0 ext{°} at the prime meridian in Greenwich, UK).

    • Analogously, celestial coordinates like Declination (similar to latitude, measured in degrees north or south of the celestial equator) and Right Ascension (similar to longitude, measured in hours, minutes, and seconds eastward from the vernal equinox) provide fixed coordinates for objects on the celestial sphere, independent of an observer's location.

  • Asterisms vs. Constellations: These terms describe perceived groupings of stars:

    • An asterism is a recognizable pattern or grouping of stars, often well-known and culturally significant (e.g., the Big Dipper). Asterisms are informal and can be parts of one or more constellations, or even include stars from different constellations.

    • A constellation is a precisely defined region of the celestial sphere. In 1928, the International Astronomical Union (IAU) officially recognized and established 88 modern constellations, each with specific boundaries, ensuring every point in the night sky belongs to exactly one constellation.

Historical Context

  • The 88 modern constellations adopted in 1928 have deep historical roots. Many derive from ancient civilizations, including Mesopotamia, Egypt, Greece, and others. These cultures used constellations for practical purposes like timekeeping, agriculture, navigation, and to imbue the sky with their myths and stories. The IAU's formalization provided a standardized system for astronomical observation worldwide.

  • Key Asterisms and Constellations - **Big Dipper** (part of Ursa Major, a prominent asterism in the Northern Hemisphere known for its seven bright stars, which can be used to locate Polaris, the North Star. Ursa Major is one of

Here are the answers to your questions:

  1. If two people live in different cities, they generally do not see the same zenith because the zenith is the point directly overhead an observer, which changes with location. Similarly, they generally do not see the same horizon, as the horizon is the imaginary plane tangent to Earth at the observer's specific location. However, they are both part of the same conceptual celestial sphere, which is an imaginary model that is infinitely large and centered on Earth, but the visible portion (celestial dome) will differ.

  2. The celestial sphere is not a physical object; it is an imaginary model or a conceptual tool used by astronomers.

  3. The stars appear to rotate around the North Celestial Pole (NCP) and the South Celestial Pole (SCP). These are the fixed points where the extended rotation axis of Earth intersects the celestial sphere.

  4. The stars appear to rotate primarily due to Earth's eastward rotation on its axis, causing them to perform an apparent daily motion from east to west across the sky. The stars themselves do move through space, but their individual movements are generally not noticeable in a single night or even over many years without precise observation; their apparent daily rotation is a reflection of Earth's motion.

  5. The Sun rises and sets because of Earth's eastward rotation on its axis. As Earth turns, different parts of its surface face towards or away from the Sun, creating the illusion that the Sun is moving across the sky.

  6. No, not all stars rise and set. Stars located near the celestial poles (known as circumpolar stars) remain visible above the horizon for an observer and appear to trace circles around the celestial pole, never rising or setting. Polaris, for example, appears almost stationary near the NCP in the Northern Hemisphere.

  7. The rotation of the Earth around the Sun (revolution) results in the apparent annual movement of the Sun across the background stars, different constellations being visible at different times of the year, and, combined with Earth's axial tilt, the changing seasons. The rotation of the Earth around its axis causes the apparent daily motion of celestial bodies, including the rising and setting of the Sun, Moon, and stars, and defines the duration of a day.

  8. "The Sun follows the same path in the sky every day." This statement is false. The Sun's path in the sky (the ecliptic) shifts subtly throughout the year due to Earth's revolution around the Sun and its axial tilt. This causes the Sun's rising and setting points on the horizon to change daily, and its maximum altitude in the sky at noon to vary with the seasons.

  9. "The axis of rotation of the Earth around itself is in a different direction compared to the axis of rotation of the Earth around the Sun." This statement is true. Earth's rotational axis is tilted by approximately 23.5 ext{°} relative to the plane of its orbit around the Sun (the ecliptic plane). The "axis of rotation around the Sun" can be interpreted as the normal (perpendicular axis) to the orbital plane, and Earth's spin axis is indeed tilted relative to this normal.

  10. The "belt" of the zodiac contains the paths of the Sun, Moon, and the major planets of our solar system (Mercury, Venus, Mars, Jupiter, and Saturn, along with the other planets).

  11. In modern astronomical terminology:

    • An asterism is a recognizable pattern or grouping of stars that is often culturally significant and informal (e.g., the Big Dipper). Asterisms can be parts of one or more constellations or even include stars from different constellations.

    • A constellation is a precisely defined region of the celestial sphere. The International Astronomical Union (IAU) officially recognized and established 88 modern constellations in 1928, each with specific boundaries, ensuring every point in the night sky belongs to exactly one constellation.

  12. "All the constellations in the sky correspond to zodiac signs." This statement is false. There are 88 officially recognized constellations, but only a subset of these (typically 12 or 13, including Ophiuchus) lie along the ecliptic and are considered zodiac constellations.

  13. No, aliens in a different galaxy would not see the same constellations that we see. Constellations are patterns formed by stars that appear close together from our specific vantage point on Earth. These stars are actually at vastly different distances. From another galaxy, the perspective would be entirely different, and our familiar patterns would be unrecognizable, or the individual stars that form our constellations might not even be resolvable.