Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Celestial Sphere
AST3018 + History of Astronomy
| Term | Definition |
|---|---|
| Celestial sphere (CS) | The Earth's axis extends to infinity which marks the north CP and south CP. The Earth's equator extends to infinity marking the the celestial equator. 0 point passes through Prime Meridian which is the Greenwich observatory. |
| Zenith | Point directly above the observer. |
| Celestial/Observer's meridian | Imaginary arc passing through the celestial poles and through the zenith. |
| Nadir | The opposite direction of the zenith. Straightdown. |
| Declination (DEC) | The CS equivalent of latitude . + is north of CE and - is south of CE. CE is 0 and poles are +/- 90. |
| Right ascension (RA) | The CS equivalent of longitude. This is said in hours, minutes, and seconds of time. Since we rotate 360 in 24 hr. 15/hour so an hour of RA is equal to 15 degrees of sky rotation. |
| Ecliptic | Apparent annual path of the Sun in the celestial sphere. Extension onto the celestial sphere of the Earth’s orbital plane about the Sun. The ecliptic is inclined 23.5 degrees with the celestial equator because the Earth axis is inclined 23.5 degrees. |
| Max altitude | Max altitude is the highest point in the sky. |
| Minimum altitude | Depends on the stars trajectory in the sky. Stars follow a path parallel to the CE. This also means that it travels equal degrees from one end of the CP to the other side. This is used to find the minimum altitude. |
| Circumpolar | If the star does not set it will stay in the sky throughout the night. |
| Vernal equinox | Point where the Sun crosses into the northern hemisphere beginning spring. This is the zero point for RA and happens when the ecliptic circle intersects the CE. Known as the first point of Aries, often identified by the symbol of the ram. 20/21 March |
| Autumnal equinox | Point where the Sun crosses from northern into the southern hemisphere in fall. This is 180 degrees for RA. September 22nd |
| Equinox | Sun is directly above the equator. Rises due east and sets due west. The daytime and nighttime is equal across the planet if Sun were a point and there was no atmospheric refraction.(Happens within a few days instead). |
| Horizon Coordinate System | Based on an observer's horizon. The zenith is directly overhead. Nadir below. The altitude goes from 0 - 90 from the pole to the zenith. Azimuth would represent longitude coordinate which goes from 0 - 360 degrees. |
| Equatorial Coordinate System | Uses info from CS such as RA and DEC. This is used to catalog stars. |
| Diurnal circles | Circular paths parallel to the celestial equator. |
| Aphelion | Farthest point happens two weeks after June solstice. (July) |
| Parahelion | Closest point which happens 2 weeks after December solstice (January). |
| Summer Solstice | Sun is overhead at Tropic of Cancer. Sun never sets above the Arctic Circle. Never rises below Antarctic circle. Greatest point north of equator. June 21st. |
| Winter Solstice | Sun is overhead at Tropic of Capricorn. Sun never rises above the Artic Circle. Never sets below Antarctic circle. |
| Approximate sunrise directions | NE in June (Summer Solstice). Due East in March and September during Equinoxes. SE in December (Winter Solstice). |
| Zodiac | Band centered on the ecliptic and encompassing all constellations that intersect the ecliptic. 13 total. |
| Altazimuth Coordinates | A system for locating celestial objects that places the observer at the center of a sphere with the zenith and nadir as its poles. It uses the ideal horizon and cardinal points (NESW) as the prinical reference point. |
| Altitude | angular distance measured upward from the horizon to the star along the star’s vertical circle (an arc forming a 90-degree angle and joining the star with the horizon). It goes from 0 to 90 degrees. |
| Annomalistic month | interval between successive passages of the moon by its perigee point (point nearest the Earth), 27.55455 days |
| Azimuth | angular distance measured from the north point on the horizon to the base of the star’s vertical circle (an arc forming a 90 degree angle and joining the star with the horizon). It is measure from the North towards the East. It goes from 0 to 360 degrees. |
| Celestial Meridian | Great circle passing through the celestial poles and the zenith of the observer |
| Constellations | The sky is divided into a total of 88 constellations that we still use today. Most of the ones we use come from Greek mythology. |
| Declination | Angular distance measured from the equator to a star along the star’s hour circle. It is + N of the Celestial Equator and -S of the Celestial Equator. Declination is given in degrees, arcminutes and arcseconds |
| Heliacal rising | first appearance of a star |
| Ideal Horizon | The edges of a plane halfway between the Zenith and the nadir. Would be located at sea level and ignore hills, low valleys, etc. |
| Insolation | total amount of solar radiation energy received on a given surface area during a given time |
| Latitude | In the Earth geographic coordinate system, measured north or south of the Equator. The North Pole has 90 degrees North latitude, the Equator has 0 degrees. The altitude of Polaris is always equal to the observer’s latitude. |
| Longitude | In the Earth coordinate system, measured east or west of a reference meridian called the Prime Meridian. |
| Lunar standstill | The moon’s rising and setting positions change along the horizon in about a month. Not fixed because of the 5.1 degrees inclination of the moon’s orbit relative to the ecliptic. The Moon’s orbital plane precesses with a period of 18.6 years. |
| Major Lunar Standstill | maximum monthly range of Moon rising and setting |
| Meridian | Circle joining the North and South Celestial pole of Earth and crossing the Equator at a 90 degree angle |
| Minor Lunar Standstill | minimum monthly range of Moon rising and setting. It will occur 9.3 years after a major lunar standstill. |
| Moon nodical period | Time that it takes for the moon to go around the celestial sphere and return to the same lunar ascending node (27.2 days) |
| Moon sidereal period | Time that it takes for the moon to return to the same position in the sky relative to the stars. The sidereal period is 27.3 days. |
| Moon synodic period | Time that it takes for the moon to return to the same lunar phase. For example from new moon to new moon. The synodic period is 29.5 days. |
| Nodes | The points of the intersection of the orbit of one body with that of another. |
| Lunar Nodes | The ascending and descending nodes of the lunar orbit refer to the points of intersection of the lunar orbit with the ecliptic. There is a regression of the lunar nodes due to a shifting of the moon orbital plane -> precession. |
| Precession | slow conical motion of the Earth’s axis of rotation about the poles of the ecliptic resulting in a motion of the celestial poles among the stars. The precession period is about 26,000 years. It causes the north celestial pole to shift westward. |
| Sidereal day | Interval between successive passages of a location on Earth by a given star. |
| Solar day | interval between successive passages of the Sun by a given place as seen from Earth. The solar day is 24 hours. It is four minutes longer than the sidereal day. |
| Standstill | The sunrise direction changes from NE at summer solstice to SE at winter solstice and back to NE again. There is a slight stopping at these two points. The word “solstice” means “standstill of the Sun” |
| Tropical year | : period of revolution of the Earth about the Sun. It is measured from Vernal Equinox to the next Vernal Equinox (365.24220 days) |
Created by:
ncam0720