Sept. 9th/11th - The paths of stars; diurnal cycle Flashcards
How do we locate objects on the celestial sphere?
Celestial coordinates: describe the precise position of a star on the celestial sphere, similar to how we use longitude + latitude to locate a city on earth
2 celestial coordinates:
- Declination
- Right Ascension
Declination (2 celestial coordinates)
- Similar to latitude:
- Similar to how latitude is parallel to Earth’s equator, lines of declination are parallel to the celestial equator
- Earth’s equator has lat = 0 deg, celestial equator has dec. 0 deg
- Latitude is labelled east or west relative to the equator??????, while declination is labelled positive or negative
- EX: North pole = 0 deg. N, North Celestial Pole has dec. = +90 deg.
Right Ascension (2 celestial coordinates)
- Similar to longitude:
- Both span vertically, north to south
- As there is no starting point for longitude, there is no starting point for right ascension
- By international treaty, longitude 0 (prime meridian) runs through Greenwich, and by convention, right ascension 0 is the line of right ascension that runs through the March equinox - Longitude is measured in degrees east or west of Greenwich, while right ascension in measured in hours east of the March equinox
- A full 360° circle around the celestial equator goes through 24 hours of right ascension, so each hour of right ascension represents an angle of 360°÷24=15°
How do we locate stars using declination/right ascension?
- DEC: + = above equator, - = below equator
- RA: each hour = 15 degrees, units of time = how long after the March equinox the object crosses the meridian
Celestial coordinates aren’t always accurate:
Move slowly relative to distant stars because they’re tied to the celestial equator - which moves gradually relative to the constellations; require continuous updating
Celestial Coordinates of the Sun:
- It takes a year for the Sun to make a full circuit of the ecliptic, which means it moves through all 24 hours of right ascension over the course of the year.
- Unlike stars, which remain essentially fixed in the patterns of the constellations on the celestial sphere, the Sun moves gradually along the ecliptic.
- The Sun therefore moves approximately one-twelfth of the way around the ecliptic each month, meaning that its right ascension changes by about 24÷12=2 hours per month.
- While RA advances steadily through the year, the Sun’s declination changes much more slowly around the solstices than around the equinoxes
Equinoxes & Solstices:
- Special moments that occur each year when earth is at particular positions in its orbit
- Correspond to the locations of the sun along the ecliptic
“Equinox” has a dual meaning:
it’s the moment in March when the sun’s path crosses the celestial equator, and the point on the ecliptic at which the sun appears to be located at that moment
How do stars move through local sky?
The path of any star through your local sky depends only on
- (1) your latitude and
- (2) the declination of the star.
The sky at the north pole - where the daily paths of stars are easiest to understand
- The daily circles of the stars keep them at constant altitudes above or below the North Polar horizon
- The altitude of any star is equal to its declination
- As a result, all stars north of the celestial equator are circumpolar at the North Pole, meaning that they never fall below the horizon.
The Sky at the Equator:
As the equatorial sky appears to turn, all star paths rise straight out of the eastern horizon and set straight into the western horizon, with the following features:
- Stars with dec=0° lie on the celestial equator and therefore rise due east, cross the meridian at the zenith, and set due west.
- Stars with dec>0° rise north of due east, reach their highest point on the meridian in the north, and set north of due west. Their rise, set, and highest point depend on their declination.
- Stars with dec<0° rise south of due east, reach their highest point on the meridian in the south, and set south of due west. For example, a star with dec=−50° rises 50° south of due east, crosses the meridian 50° to the south of the zenith—that is, at an altitude of 90°−50°=40° in the south—and sets 50° south of due west.
- Because exactly half of any star’s daily circle lies above the horizon, every star at the equator is above the horizon for exactly half of each sidereal day, or just under 12 hours (and below the horizon for the other half of the sidereal day).
The celestial equator always extends from due east on your horizon to due west on your horizon, crossing the meridian at an altitude of…
90° minus your latitude.
How Does The Sun Move Through Local Sky?
The Sun’s path on any particular day depends only on its declination and your latitude - because the Sun’s declination changes over the course of the year, the Sun’s path also changes
During Equinoxes - How Does The Sun Move Through Local Sky?
the Sun is on the celestial equator (dec=0°) and therefore follows the celestial equator’s path: It rises due east, crosses the meridian at altitude 50° in the south, and sets due west; above the horizon for 12 hours
