4.1 Earth and Sky
3 min read•june 12, 2024
Earth's coordinate system and celestial are essential for mapping our planet and navigating the night sky. These systems use and on Earth, and and in the sky, to pinpoint locations with precision.
Understanding these coordinates helps us grasp Earth's rotation and its effects on celestial observations. As our planet spins, stars appear to move across the sky, creating day and night cycles and influencing which celestial objects we can see at different times.
Earth's Coordinate System
Latitude and longitude on Earth
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- Latitude measures angular distance north or south of
- Ranges from 0° at equator to 90° at poles (North Pole, South Pole)
- 0° to 90° N for Northern Hemisphere (Arctic Circle)
- 0° to 90° S for Southern Hemisphere (Antarctic Circle)
- Longitude measures angular distance east or west of
- Ranges from 0° to 180° E or 0° to 180° W ()
- Prime defined as 0° longitude, passes through Greenwich, England
- Combining latitude and longitude specifies any point on Earth's surface
- Unique coordinate pair (40°N, 74°W for New York City)
- Used for navigation, mapping, and geolocation (GPS)
Celestial Coordinate System
Right ascension and declination in sky
- is imaginary sphere surrounding Earth, celestial objects projected onto it
- is projection of Earth's equator onto
- are projection of Earth's poles onto celestial sphere (Celestial North Pole, Celestial South Pole)
- () measures angular distance north or south of
- Ranges from 0° at celestial equator to 90° at
- 0° to 90° N for Northern Celestial Hemisphere ( at ~90°N)
- 0° to 90° S for Southern Celestial Hemisphere ( at ~60°S)
- () measures angular distance eastward along celestial equator from
- Ranges from 0h to 24h ( at ~5h, at ~11h)
- Vernal equinox defined as 0h right ascension, point where Sun crosses celestial equator in March
- Combining right ascension and declination specifies any point on celestial sphere
- Unique coordinate pair ( at = 6h 45m, = -16° 43')
- Used for locating and tracking celestial objects (stars, planets, galaxies)
Celestial motion vs Earth's rotation
- Earth rotates west to east about its axis once every 24 hours
- Rotation period defines length of day
- Rotation axis tilted 23.5° relative to orbital plane ()
- Celestial objects appear to rise in east, move across sky, and set in west due to Earth's rotation
- Apparent motion opposite of Earth's rotation (Sun rises in east, sets in west)
- Different parts of sky visible at different times of night and year
- objects near celestial poles never rise or set
- Appear to circle celestial poles due to Earth's rotation ( around Celestial North Pole)
- Number of circumpolar objects depends on observer's latitude
- is point directly overhead on celestial sphere
- Changes with observer's location and time
- is passing through celestial poles and
- Objects reach highest point in sky when crossing meridian ()
- Meridian divides sky into eastern and western halves
Horizon Coordinate System
- measures angular distance along the horizon from north, increasing clockwise
- measures angular distance above the horizon
- Used for describing the position of celestial objects relative to an observer's local horizon
Earth's Motions and Time
- is the time it takes for Earth to complete one rotation relative to the fixed stars (approximately 23 hours and 56 minutes)
- is the time between two consecutive noons, slightly longer than a due to Earth's orbital motion (24 hours)
- is the slow, circular motion of Earth's rotational axis, completing one cycle every 26,000 years
Historical Models of the Universe
- proposed a geocentric model with Earth at the center of the universe
- introduced a heliocentric model with the Sun at the center of the solar system, revolutionizing our understanding of celestial motions
Key Terms to Review (44)
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.
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: Circumpolar refers to celestial objects or stars that never set below the horizon, but instead appear to rotate around the celestial pole. These objects are visible in the night sky throughout the entire year, making a complete circle around the pole over the course of a day.
Coordinates: Coordinates are numerical values that define a specific point in a space or on a surface. In astronomy, coordinates are used to pinpoint the location of celestial objects in the sky.
Copernicus: Copernicus was a Renaissance-era astronomer who proposed a heliocentric model of the solar system, where the Earth and other planets orbit around the Sun. This revolutionary idea challenged the prevailing geocentric view that had dominated Western thought for centuries.
Culmination: Culmination refers to the highest or final point of a process, event, or development, marking the climax or completion of something. It represents the point at which a series of actions or events reaches its highest or most decisive stage.
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.
Equator: The equator is an imaginary line that divides the Earth into the Northern and Southern hemispheres. It is an important reference point in the study of Earth and the sky, as it serves as the basis for many geographical and astronomical concepts.
Foucault: Foucault was a French physicist who demonstrated the Earth's rotation using a pendulum, known as Foucault's pendulum. His experiments provided direct visual evidence of Earth's rotation in the mid-19th century.
Great circle: A great circle is the largest possible circle that can be drawn on a sphere, such as Earth, with its center coinciding with the center of the sphere. It represents the shortest path between any two points on the surface of the sphere.
International Date Line: The International Date Line is an imaginary line that runs from the North Pole to the South Pole, roughly following the 180th meridian of longitude. It is the line that separates one calendar day from the next, allowing for the tracking of time and date across the globe.
Latitude: Latitude is the north-south position of a location on the Earth's surface, measured in degrees north or south of the equator. It is a crucial concept in understanding the Earth's geography, climate, and astronomical observations.
Longitude: Longitude is the east-west position of a point on the Earth's surface, measured in degrees east or west from the prime meridian, which passes through the Royal Observatory in Greenwich, England. It is one of the two geographic coordinates that, along with latitude, define the position of any location on the planet.
Meridian: A meridian is an imaginary line that runs from the North Pole to the South Pole and passes through a specific point on Earth's surface. It is used in astronomy to determine the position of celestial bodies.
Meridian: A meridian is an imaginary line on the Earth's surface that runs from the North Pole to the South Pole, passing through a specific point. Meridians are used to define longitude and provide a reference for measuring the east-west position of a location on the planet.
Orion: Orion is a prominent and easily recognizable constellation in the night sky, named after the Greek mythological hunter. It is one of the most conspicuous and distinctive star patterns, featuring a distinctive hourglass or belt-and-sword shape that is visible throughout the world.
Polaris: Polaris, also known as the North Star, is a prominent star in the northern celestial hemisphere. It holds great significance in the context of ancient astronomy, Earth's sky, the seasons, and the brightness of stars.
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.
Prime Meridian: The prime meridian is an imaginary line that runs from the North Pole to the South Pole, passing through Greenwich, England. It is the reference line from which longitude is measured, with locations east and west of the prime meridian designated as having positive or negative longitude values, respectively.
Ptolemy: Ptolemy was a prominent ancient Greek astronomer, mathematician, and geographer who lived in Alexandria, Egypt during the 2nd century AD. He is best known for his comprehensive model of the universe, which placed the Earth at the center and described the motions of the Sun, Moon, and planets.
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 day: A sidereal day is the time it takes for Earth to rotate once relative to distant stars, approximately 23 hours, 56 minutes, and 4 seconds. It is different from a solar day, which is based on the position of the Sun in the sky.
Sidereal Day: A sidereal day is the time it takes for the Earth to complete one rotation on its axis, as measured by the position of distant stars in the sky. It is slightly shorter than a solar day, which is the time it takes for the Earth to complete one rotation relative to the Sun.
Sirius: Sirius, also known as the Dog Star, is the brightest star in the night sky. It is a binary star system located in the constellation Canis Major, approximately 8.6 light-years from Earth. Sirius has been an important astronomical object throughout human history, with its prominence in the night sky and its significance in various cultural and religious traditions.
Solar day: A solar day is the time it takes for Earth to rotate once on its axis relative to the Sun, resulting in a full cycle from one noon to the next. It averages about 24 hours but can vary slightly due to Earth's elliptical orbit and axial tilt.
Solar Day: A solar day is the length of time it takes for the Earth to complete one full rotation on its axis, as measured by the position of the sun in the sky. It is the fundamental unit of time used to measure the duration of a day on Earth.
Southern Cross: The Southern Cross, also known as Crux, is a prominent constellation visible in the night sky of the Southern Hemisphere. It is one of the most recognizable and iconic constellations, serving as a valuable navigational tool for those in the southern latitudes.
Ursa Major: Ursa Major, also known as the Big Dipper or the Great Bear, is a prominent constellation in the northern celestial hemisphere. It is one of the most easily recognizable star patterns in the night sky and has been observed and studied by astronomers and stargazers for centuries.
Ursa Minor: Ursa Minor, also known as the Little Bear or the North Star, is a prominent constellation in the northern celestial hemisphere. It is one of the most recognizable constellations in the night sky and holds great significance in both the study of Earth and the sky, as well as the properties of galaxies.
Vernal Equinox: The vernal equinox, also known as the spring equinox, is an astronomical event that occurs twice a year when the sun crosses the celestial equator, marking the beginning of spring in the northern hemisphere and the beginning of fall in the southern hemisphere. It is a significant event in the annual cycle of seasons and has been observed and celebrated by various cultures throughout history.
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.