2.1 The Sky Above
4 min read•june 12, 2024
The is a powerful tool for understanding the sky. It's an imaginary dome surrounding Earth where we project celestial objects. This concept helps us map stars, , and other cosmic wonders, making it easier to navigate the night sky.
Coordinate systems are crucial for pinpointing celestial objects. The equatorial system uses and , while the horizontal system relies on and . These systems allow astronomers to precisely locate and track objects in the vast expanse of space.
Celestial Sphere and Coordinate Systems
Elements of celestial sphere
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- represents an imaginary sphere surrounding Earth with celestial objects projected onto its surface, providing a useful framework for mapping and describing positions of stars and other objects in the sky
- consist of the North and South , which are points where Earth's axis of rotation intersects the celestial sphere, located directly above Earth's North and South poles (Polaris, Southern Cross)
- is the projection of Earth's equator onto the celestial sphere, dividing it into northern and southern hemispheres
- represents the point directly overhead for an observer, while is the point directly opposite the , below the observer's feet
- is the visible boundary between Earth and sky, dividing the celestial sphere into upper (visible) and lower (invisible) hemispheres (sunrise, sunset)
Coordinate systems for celestial mapping
- is based on the and celestial poles
- (RA) measures the angle eastward along the celestial equator from the vernal equinox in hours, minutes, and seconds (24 hours in a full circle)
- (Dec) measures the angular distance north or south of the celestial equator in degrees, arcminutes, and arcseconds (-90° to +90°)
- is based on the observer's local
- Altitude measures the angular distance above the horizon (0° to 90°)
- Azimuth measures the angular distance clockwise from due north along the horizon (0° to 360°)
Apparent motions of stars
- Daily motion causes stars to appear to rise in the east, move across the sky, and set in the west due to Earth's rotation from west to east
- Annual motion results in stars appearing to shift slightly westward each night at the same time due to Earth's revolution around the Sun, causing different constellations to be visible at different times of the (zodiac constellations)
- never rise or set, appearing to circle the celestial poles, with their visibility depending on the observer's latitude (Ursa Major, Southern Cross)
- causes a slow change in the direction of Earth's axis over a period of about 26,000 years, affecting the positions of celestial poles and equinoxes
Sun, Moon, and planets vs fixed stars
- Sun appears to move eastward along the (path of the Sun on the celestial sphere) over the course of a year, causing the changing seasons and the variation in the length of day and night (solstices, equinoxes)
- Moon moves eastward relative to the stars, completing a full orbit in about 27.3 days (), with its phases caused by the changing angle between the Sun, Earth, and Moon (new moon, full moon)
- Planets exhibit , a periodic westward motion relative to the stars, caused by the combined motions of Earth and the planets around the Sun (Mars, Jupiter)
- is the apparent change in position of nearby stars relative to more distant stars as Earth orbits the Sun, used to measure distances to stars
Constellations in observation and navigation
- Constellations are recognized patterns of stars in the sky, with 88 official constellations defined by the (IAU) (Orion, Ursa Major)
- Constellations have historical and cultural significance, having been used for storytelling, mythology, and navigation by ancient civilizations (Greek mythology, Polynesian navigation)
- In modern astronomy, constellations provide a reference frame for locating and describing celestial objects, and are used in naming stars (Alpha Centauri) and deep-sky objects (M31 in Andromeda)
- Certain constellations and stars can be used for navigation to determine direction and latitude (Polaris for north, Crux for south)
- Asterisms are prominent star patterns that are not official constellations but are often used as sky markers (Big Dipper, Summer Triangle)
Observing conditions and measurements
- from artificial lighting can interfere with astronomical observations by reducing the visibility of faint celestial objects
- refers to the steadiness and clarity of the atmosphere, affecting the quality of astronomical observations
- is a measure of the brightness of a celestial object as seen from Earth, with lower numbers indicating brighter objects
Key Terms to Review (40)
Altitude: Altitude refers to the vertical distance of an object or location above a reference point, typically the Earth's surface or sea level. This term is particularly important in the context of understanding the sky above and the relationship between the Earth and the sky.
Apparent Magnitude: Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. It is a logarithmic scale that quantifies the observed luminosity of stars, planets, and other astronomical objects in the night sky.
Apparent magnitudes: Apparent magnitude measures the brightness of a celestial object as seen from Earth. It is a logarithmic scale where lower values indicate brighter objects.
Asterism: An asterism is a recognizable group of stars within a larger constellation. It is a distinctive pattern of stars that is smaller and more noticeable than the full constellation it is a part of.
Astronomical Seeing: Astronomical seeing refers to the quality of the Earth's atmosphere and its impact on the clarity and sharpness of celestial observations. It describes the blurring and distortion of light from astronomical objects caused by turbulence and variations in the refractive index of the atmosphere.
Azimuth: Azimuth is an angular measurement in a horizontal plane, typically used to specify the direction of a celestial object or a direction on Earth. It is the horizontal coordinate that specifies the east-west position of a point in the sky or on the Earth's surface.
Big Telescope Altazimuth: Big Telescope Altazimuth mounts are used for large telescopes and offer two-axis movement: altitude (up/down) and azimuth (side to side). This type of mount simplifies the design and alignment process for large astronomical observatories.
Celestial equator: The celestial equator is an imaginary projection of Earth's equator into space. It divides the sky into the northern and southern celestial hemispheres.
Celestial Equator: The celestial equator is an imaginary line in the celestial sphere that corresponds to the Earth's equator, dividing the sky into northern and southern hemispheres. It serves as a key reference point for understanding the motions of celestial objects and their positions in the night sky.
Celestial poles: Celestial poles are the two imaginary points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The North Celestial Pole is near Polaris, while the South Celestial Pole does not have a bright star marking its position.
Celestial Poles: The celestial poles are the two points in the celestial sphere where the Earth's axis of rotation intersects the sky. They are the fixed points around which the entire celestial sphere appears to rotate as the Earth spins on its axis, making them crucial reference points for understanding the motion of celestial objects in the night sky.
Celestial sphere: The celestial sphere is an imaginary sphere that extends infinitely into space with Earth at its center, used to map the positions of stars and planets. Astronomers use it to simplify the complex motions of celestial bodies for observation and navigation.
Celestial Sphere: The celestial sphere is an imaginary, three-dimensional sphere that surrounds the Earth, upon which all celestial objects and events appear to be projected. It serves as a conceptual framework for understanding the motions and positions of objects in the sky, and is a fundamental concept in the study of astronomy.
Circumpolar Stars: Circumpolar stars are celestial objects that never set below the horizon from a given location on Earth. These stars appear to revolve around the celestial pole, making complete circles in the sky each day, and are always visible throughout the night from their observation point.
Circumpolar zone: The circumpolar zone is a region of the sky where stars never set below the horizon, appearing to circle around the celestial pole. This zone varies depending on the observer's latitude on Earth.
Constellation: A constellation is a group of stars that forms a recognizable pattern in the night sky, often named after mythological figures, animals, or objects. These patterns are used for navigation and to help astronomers organize the sky into manageable sections.
Declination: Declination is the angular distance of a celestial object north or south of the celestial equator. It is measured in degrees, with positive values indicating north and negative values indicating south.
Declination: Declination is the angular distance of a celestial object north or south of the celestial equator. It is one of the two coordinates used to locate objects in the sky, the other being right ascension. Declination is a crucial concept in understanding the movement and positioning of celestial bodies, including those observed on Earth.
Ecliptic: The ecliptic is the apparent path that the Sun traces out in the sky over the course of a year as observed from Earth. It also represents the plane of Earth's orbit around the Sun.
Ecliptic: The ecliptic is the apparent path that the Sun appears to take across the sky over the course of a year, as viewed from Earth. It is the projection of Earth's orbit around the Sun onto the celestial sphere, and it serves as a reference plane for many important astronomical phenomena.
Equatorial Coordinate System: The equatorial coordinate system is a celestial coordinate system that uses the celestial equator and celestial poles as the fundamental plane and points of reference, respectively. This system is widely used in astronomy to precisely locate and describe the positions of celestial objects in the sky.
Geocentric: The geocentric model is an ancient astronomical theory that places Earth at the center of the universe, with all other celestial bodies orbiting it. This model was widely accepted until the heliocentric model gained prominence during the Renaissance.
Horizon: The horizon is the line where the Earth and sky appear to meet. It is the boundary separating the visible sky from the ground or sea, depending on your location.
Horizon: The horizon is the apparent line that separates the earth from the sky, the line that marks the farthest visible boundary of the earth's surface. It is the circular line that divides all visible directions into two categories: those that intersect the earth's surface, and those that do not.
Horizontal Coordinate System: The horizontal coordinate system is a celestial coordinate system used to locate objects in the sky based on their angular positions relative to the observer's local horizon. It is a fundamental system for describing the apparent motions of celestial bodies as they move across the sky.
International Astronomical Union: The International Astronomical Union (IAU) is the international organization of professional astronomers and planetary scientists. It is responsible for the naming and definition of celestial bodies, as well as the promotion of astronomy research and education worldwide.
Light Pollution: Light pollution refers to the excessive, misdirected, or obtrusive use of artificial (usually outdoor) light, which has a negative impact on the natural environment, particularly the night sky. It is a growing concern in many parts of the world, affecting astronomical observations, wildlife, and human health.
Nadir: The nadir is the point on the celestial sphere directly below an observer's location, directly opposite the zenith. It is the lowest point in the sky, the point where the observer's vertical line intersects the celestial sphere.
Planets: Planets are celestial bodies that orbit a star, such as the Sun, and have cleared their orbit of other debris. They do not produce their own light but reflect the light of their parent star.
Precession: Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.
Precession: Precession is the gradual shift in the orientation of an object's rotational axis in relation to a fixed reference frame. This phenomenon occurs in various celestial bodies, including Earth, and has significant implications for the study of astronomy, timekeeping, and the calendar.
Retrograde motion: Retrograde motion is the apparent backward movement of a planet against the background of stars. This phenomenon occurs when Earth, which moves faster in its orbit, overtakes and passes an outer planet.
Retrograde Motion: Retrograde motion refers to the apparent backward or westward movement of a celestial body across the sky, relative to the fixed stars, as observed from Earth. This phenomenon is observed for certain planets in our solar system and can provide important insights into the dynamics of planetary motion.
Right ascension: Right ascension (RA) is the angular distance measured eastward along the celestial equator from the vernal equinox. It is analogous to longitude on Earth but measured in hours, minutes, and seconds.
Right Ascension: Right ascension is one of the celestial coordinates used to locate a point on the celestial sphere. It is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle passing through the point.
Sidereal Month: The sidereal month is the time it takes for the Moon to complete one full orbit around the Earth, as measured against the fixed background of distant stars. It is the time it takes for the Moon to return to the same position relative to the stars.
Stellar Parallax: Stellar parallax is the apparent shift in the position of a nearby star relative to more distant stars, caused by the Earth's annual motion around the Sun. It is a fundamental concept in astronomy that allows for the direct measurement of the distances to nearby stars.
Year: A year is the period of time it takes for a planet to complete one orbit around its star. On Earth, a year is approximately 365.25 days long.
Zenith: The zenith is the point in the sky that is directly above an observer's position on the Earth's surface. It represents a 90-degree angle from all points on the horizon.
Zenith: The zenith is the point directly overhead in the sky, directly above the observer's location. It is the highest point that an object can reach in the sky from the perspective of the observer on Earth.