Astro 113: The Celestial Sphere - Lecture Slides PDF

Astro 113 Slide 3 Topics The Celestial Sphere Geocentric Observations Ecliptic, Equinoxes, Solstices Seasons Precession Retrograde Motion Observations Naked eye: Observation with no equipment Can help us with: Navigation, cycle of days, rhythm of seasons,...

Astro 113 Slide 3 Topics The Celestial Sphere Geocentric Observations Ecliptic, Equinoxes, Solstices Seasons Precession Retrograde Motion Observations Naked eye: Observation with no equipment Can help us with: Navigation, cycle of days, rhythm of seasons, movement of earth Many phenomena require telescope and other instruments: What stars are made of, the distance of galaxies and etc. Constellation When ancient people looked at the stars, they imagined that groupings of stars traced out pictures in the sky. We refer to them as constellations (Latin: group of stars) As seen from the Earth Example: Big Dipper, which is part of the large Constellation Usra Major. https://theplanets.org/big-dipper Note The Earth rotates from west to east, making one complete rotation every 24 hours, which is why there is daily cycle of day and night. Because of this rotation, stars appear to us to rise in the east and set in the west, as do the Sun and Moon. The Celestial Sphere The Sun and Moon seem to follow consistent paths as they travel across the sky. The stars also seem to move across the sky, but their positions remain constant relative to each other. Planets resemble stars in appearance but shift their positions relative to the stars. The term "planet" originates from a Greek word meaning "wanderer." The Celestial Sphere It is not surprising that the ancient Greeks, along with many other ancient civilizations, believed the Earth was stationary while the Sun, Moon, planets, and stars revolved around it. This concept, known as the Geocentric Model, places the Earth at the center of the universe. They envisioned the stars as being fixed to a rotating sphere, called the Celestial Sphere, which rotates the Earth. The Celestial Sphere The concept of the celestial sphere is still used today as a tool for mapping the positions of stars, galaxies, and other celestial objects. This system is quite similar to how we map locations 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, passing the center of the Earth, and perpendicular to the axis of rotation. http://the- stewardship.org/research The Celestial Sphere We locate the stars 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: The point in the sky directly overhead an observer anywhere on From “Universe” by Freedman Earth. The Celestial Sphere The Earth is at the center of the celestial Sphere. We treat the Earth as if it is not moving (stationary) and that celestial objects rotate around us. The celestial poles are the points on the celestial sphere that represent the extension of the Earth's axis of rotation into space. These poles are directly above the Earth's North Pole and South Pole, respectively, and are aligned with Earth's axis of rotation. Note: stars sufficiently near the celestial poles revolve around the pole, never rising or setting. Such starts are called circumpolar. The Celestial Sphere Latitude is a measure of how far north or south a point on the surface of the Earth is. Lines of latitude run around the Earth parallel to the Equator. Latitude is measured in degrees, ranging 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. Latitude Note: Lines of Latitude are also known as a Parallel. Equator The Celestial Sphere Longitude measures a location's position east or west of the Prime Meridian, which is the 0° reference line running through Greenwich, England (East or West). Each meridian runs from the North Pole to the South Pole. Lines of Longitude are also known as a Meridians, and unlike parallels (latitude), they converge at the poles and are widest at the equator Longitude Line of Longitude Equator Prime Meridian The Celestial Sphere From our perspective, the celestial sphere appears to rotate around the Earth once every day. After one full rotation of the Earth, a star on the celestial sphere will return to the same position above a specific point on Earth. This time period is known as a sidereal day. This is NOT the same time interval that we normally call a day Our day is defined as a solar day, which is the time it takes for the Sun to return to the same position above a specific location on Earth. Question: The Sidereal Day is about 4 minutes shorter than the Solar Day, why? The Celestial Sphere Key Concepts Celestial Sphere: an imaginary sphere with earth as its center. North and South celestial Poles. Celestial Equator. Coordinates: Declination and Right Ascension. Vernal Equinox. The Celestial Sphere We locate a star by its position on the celestial sphere. One coordinate, similar to latitude, represents the angle of a star relative to the celestial equator. This is known as Declination. It is abbreviated Decl. Note: A positive declination indicates a position Dec. toward the North Celestial Pole, while a negative declination points toward the South Celestial Pole. The units of Decl. is in Degrees, arcminutes and arcseconds. Example: Decl. is 90° at North Celestial pole And -90° at South Celestial pole. The Celestial Sphere One other coordinate to locate to locate astronomical objects is Right Ascension (in analogy to longitude), which is the angle of a star from a position known as the Vernal Equinox. Vernal Equinox Right ascension is abbreviated R.A. R.A is positive toward the East. R.A. The units of R.A is usually given in hours, minutes, and seconds.  The Celestial Sphere Ecliptic Plane: The plane in which the Earth orbits the Sun. Ecliptic : The Sun's apparent path across the celestial sphere, as seen from Earth. Note: 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.  The Celestial Sphere Equinoxes: The points where the ecliptic plane crosses the celestial equator. At the time of year when the Sun is at the equinox the length of a day and night are the same. Equinox is from the Latin for “equal night”. So people refer to the Equinoxes both as a position in the Celestial sphere and as a time of the year.  The Celestial Sphere The Vernal Equinox is one of the two points in the year when the Sun crosses the celestial equator, making day and night of approximately equal length. It 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. At the time of the Vernal Equinox, the Earth's axis is not tilted toward or away from the Sun, resulting in nearly equal amounts of sunlight being distributed across both hemispheres. The Autumnal equinox, which marks the beginning of Autumn, is near September 22. The Celestial Sphere A Solstice occurs when the Sun is at its greatest distance from the equatorial plane. Summer Solstice: when the Sun reaches its highest point in the sky relative to the equator. It is the day in the Northern Hemisphere with the longest period of daylight (which is around June 21). Winter Solstice : when the Sun reaches its lowest position in the sky relative to the equator. It is the day in the Northern Hemisphere with the shortest amount of daylight (which is around December 21). Winter and Summer The Seasons 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 Celestial Sphere Note: 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. However, 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 Dec.) of an object change with time. The Celestial Sphere The Earth behaves like a spinning top, and its axis of rotation wobbles over time. The Celestial Sphere The North Celestial pole is currently near the star Polaris (the North Star). The Celestial Sphere Therefore, when providing the coordinates of a celestial object, it is essential to specify the epoch, which indicates the date for which the coordinates are accurate. Star catalogs are updated periodically to account for changes. Most modern catalogs and star charts use the epoch 2000 (i.e., accurate as of January 1, 2000), and these remain sufficiently precise for several decades. Angle Units We normally measure angles in Degrees (°). Astronomers often express the fractions of a degree in minutes of arc and seconds of arc. We can further subdivide the arcmin into seconds of arc. 1 degree = 60 minutes of arc = 60 arcmin = 60’. 1 arcmin = 60 seconds of arc = 60 arcsec = 60”. So 1 degree = 3600 arcsec = 3600”. Note: There are 360° in a full circle. Example: The Latitude of Saskatoon is 52.166° N. 1 degree = 60 minutes of arc = 60 arcmin = 60’.  60  0.166  0.166     10 So  1  The Latitude of Saskatoon can be written 52° 10’ N. Angle Units Angles are sometimes measured in units of Hours, Minutes, and Seconds, with Right Ascension being one such example. Since the Earth completes one full rotation in 24 hours, Right Ascension is measured in hours. Therefore 24 h = 360°.  360  1h 1h    15  24 h  The hours are further divided into minutes and seconds. 1 h = 60 minutes, and 1 minute = 60 seconds. Note: These minutes and seconds are not the same as the arcmin and arcsec that divide up a degree!! Example What is the angle in the sky (in degrees) 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′′ ?