Celestial Sphere & Earth's Motion Concepts

• Topics include: The Celestial Sphere, Geocentric Observations, Ecliptic, Equinoxes, Solstices, Seasons, Precession, and Retrograde Motion.
• Naked eye observations are observations with no equipment.
• Naked eye observations can help with navigation, cycles of days, the rhythm of seasons, and the movement of Earth.
• Many phenomena require a telescope and other instruments, such as research on what stars are made of, the distance of galaxies, etc.
• When ancient people looked at the stars, they imagined groupings of stars traced out pictures in the sky.
• These groupings are referred to as constellations.
• Constellation is Latin for group of stars.
• The Big Dipper is part of the larger constellation Ursa Major.
• The Earth rotates from west to east, making one complete rotation every 24 hours, explaining the daily cycle of day and night.
• Due to the rotation of Earth, to observers, stars, the Sun and Moon appear to rise in the east and set in the west.

The Celestial Sphere

• The Sun and Moon seem to follow consistent paths as they travel across the sky.
• Positions of the stars remain constant relative to each other, even though they seem to move across the sky.
• Planets resemble stars but shift positions relative to the stars.
• The term "planet" originates from a Greek word meaning "wanderer".
• The ancient Greeks and early civilizations believed the Earth was stationary while the Sun, Moon, planets, and stars revolved around it.
• The Geocentric Model places the Earth at the center of the universe.
• Ancient civilizations envisioned the stars as being fixed to a rotating sphere, called the Celestial Sphere, which rotates the Earth.
• The concept of the celestial sphere is still used today as a tool for mapping the positions of stars, galaxies, and other celestial objects.
• Mapping the celestial sphere is similar to how locations are mapped on the Earth's surface.
• The Earth rotates on its axis through a line drawn through the poles.
• The Equator is defined by a plane, which passes the center of the Earth, and is perpendicular to the axis of rotation.
• Stars are located on an imaginary Celestial Sphere.
• The North and South Celestial Poles are the points where the Earth's axis of rotation intersects the celestial sphere.
• The Celestial Equator lies in the same plane as the Earth's equator.
• Zenith is the point in the sky directly overhead for an observer anywhere on Earth.
• The Earth is thought to be at the center of the celestial Sphere.
• The Earth is treated as if it is stationary while celestial objects rotate around Earth.
• The celestial poles represent the extension of the Earth's axis of rotation into space.
• The poles are directly above the Earth's North Pole and South Pole, and are aligned with Earth's axis of rotation.
• Stars sufficiently near celestial poles revolve around the poles, and are called circumpolar.
• Latitude measures how far north or south a point is on the surface of the Earth.
• Lines of latitude run around the Earth parallel to the Equator.
• Latitude is measured in degrees from 0° at the equator to 90° at the poles (north or south).
• Values are positive in the Northern Hemisphere and negative in the Southern Hemisphere.
• Lines of Latitude are also known as a Parallel.
• Longitude measures a location's position east or west of the Prime Meridian.
• The Prime Meridian is the 0° reference line running through Greenwich, England.
• Each meridian runs from the North Pole to the South Pole.
• Lines of Longitude are also known as Meridians.
• Unlike parallels (latitude), meridians converge at the poles and are widest at the equator.
• The celestial sphere appears to rotate around the Earth once every day.
• A star on the celestial sphere returns to the same position above a specific point on Earth after one full rotation of the Earth.
• This time period is known as a sidereal day and is not the same time interval as what is normally called a day.
• A solar day is the time it takes for the Sun to return to the same position above a specific location on Earth.
• A sidereal day is about 4 minutes shorter than the Solar Day.
• Key concepts include celestial sphere, north and south celestial poles, celestial equator, declination and right ascension, and vernal equinox.
• The celestial sphere is an imaginary sphere with earth as its center.
• Declination represents the angle of a star relative to the celestial equator and similar to latitude.
• Declination is abbreviated Decl.
• A positive declination indicates a position towards the North Celestial Pole.
• A negative declination point towards the South Celestial Pole
• Declination units include Degrees, arcminutes and arcseconds.
• Decl. is 90° at North Celestial pole and is -90° at South Celestial pole.
• Right Ascension helps locate astronomical objects and is the angle of a star from a position known as the Vernal Equinox, and similar to longitude.
• Right ascension is abbreviated as R.A.
• R.A is positive toward the East.
• Right Ascension is usually given in hours, minutes, and seconds.
• Ecliptic Plane: It is the plane in which the Earth orbits the Sun.
• Ecliptic: It is the Sun's apparent path across the celestial sphere, as seen from Earth.
• The ecliptic plane is different from the celestial equator because Earth's axis is tilted by about 23.5° in relation to its orbital plane.
• Equinoxes are the points where the ecliptic plane crosses the celestial equator.
• At the equinox, the length of a day and night are the same.
• Equinox is from the Latin for “equal night".
• The Equinoxes are a position in the Celestial sphere and a time of the year.
• The Vernal Equinox is when the Sun crosses the celestial equator, making day and night of approximately equal length.
• The Vernal equinox marks the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
• Vernal means “spring” in Latin.
• This event occurs around March 20th or 21st each year when the Earth's axis is not tilted toward or away from the Sun.
• The Autumnal equinox, marks the beginning of Autumn, and occurs near September 22.
• A Solstice occurs when the Sun is at its greatest distance from the equatorial plane.
• Summer Solstice is when the Sun reaches its highest point in the sky relative to the equator.
• The Summer Solstice is the day in the Northern Hemisphere with the longest period of daylight, and occurs around June 21.
• Winter Solstice occurs when the Sun reaches its lowest position in the sky relative to the equator.
• The Winter Solstice is the day in the Northern Hemisphere with the shortest amount of daylight, and occurs around December 21.
• Seasons change due to Earth's axial tilt.
• Summer is warmer than winter for two reasons:
• Sunlight is more concentrated when it is directly overhead.
• Sunlight travels through a thinner layer of the atmosphere when it is overhead.
• The axis of rotation of the Earth is not constant.
• Precession refers to the gradual change in the orientation of the rotational axis of a spinning object.
• The Earth's axis of rotation undergoes precession due to the gravitational influence of the Sun and the Moon.
• The celestial poles are determined by the points where the Earth's axis of rotation intersects the celestial sphere.
• These positions change over time, which causes the celestial coordinates (R.A. and Decl.) of an object change with time.
• The Earth behaves like a spinning top, and its axis of rotation wobbles over time.
• The North Celestial pole is currently near the star Polaris (the North Star).
• Providing the coordinates of a celestial object requires specifying the epoch, which indicates the date for which the coordinates are accurate.
• Star catalogs are updated periodically to account for changes.
• Modern catalogs and star charts use the epoch 2000 (i.e., accurate as of January 1, 2000).
• These remain sufficiently precise for several decades.
• Angles are measured in Degrees (°).
• Astronomers often express the fractions of a degree in minutes of arc and seconds of arc.
• Subdivision of arcmin leads to seconds of arc.
• 1 degree = 60 minutes of arc = 60 arcmin = 60'.
• 1 arcmin = 60 seconds of arc = 60 arcsec = 60”.
• 1 degree = 3600 arcsec = 3600".
• There are 360° in a full circle.
• The Latitude of Saskatoon can be written 52° 10' N and is 52.166° Ν.
• 1 degree = 60 minutes of arc = 60 arcmin = 60'.
• 0.166° = 0.166°× (60'/1°) = 10'.
• Angles are measured in units of Hours, Minutes, and Seconds, with Right Ascension being one such example.
• Right Ascension uses hours to measure angles.
• Since the Earth completes one full rotation in 24 hours, Right Ascension is measured in hours.
• 24 h = 360°.
• 1h = 1 h x (360°/ 24 h) =15°.
• One hour is 60 minutes and 1 minute is 60 seconds.
• Hours, minutes, and seconds, used for right ascension, are not the same as the arcmin and arcsec that divide up a degree.
• An example angle in the sky is calculated between a star with Right Ascension (R.A.) (0h, 0m, 0s) Declination (Decl.) (20°, 5′, 0′), and a second star with R.A. (0h, 0m, 0s) and (Decl.) (30°, 0′, 0′).

Explore the celestial sphere, Earth's orbit, and coordinate systems. Understand declination, right ascension, equinoxes, and solstices. Learn about precession and coordinate epoch for astronomical observations.