section5 lec2

The Night sky: 

The Night sky Celestial sphere (real?) Celestial equator Celestial pole Celestial coordinate system (RA & Dec) Ecliptic Equinox Solstice Zodiac Zenith Meridian Horizon

The Night sky: 

The Night sky Spring equinox Fall equinox Winter solstice Summer solstice

The Night sky: 

The Night sky Zodiac

The Night sky: 

The Night sky Horizon Meridian

Equatorial coords.: 

Equatorial coords. Right Ascension (RA): Equivalent to longitude Measured in hours (24 hrs = 360 degree or 1hr = 15 degree) increase from west to east zero point is spring equinox (around Pisces)

Equatorial coords.: 

Equatorial coords. Declination (Dec) Equivalent to latitude Measured in degrees zero point on the celestial equator North pole: 90 deg.; South pole: -90 deg

Equatorial coords.: 

Equatorial coords.

Equatorial coords.: 

Equatorial coords. The coordinate system is fixed on the celestial sphere The coordinates of stars and constellations don’t change The coordinates of the Sun do change (why?) E.g.: Sun’s RA Dec spring summer fall winter

Equatorial coords.: 

Equatorial coords.

Equatorial coords.: 

Equatorial coords. The coordinate system is fixed on the celestial sphere The coordinates of stars and constellations don’t change The coordinates of the Sun do change (why?) E.g.: Sun’s RA/hr Dec/deg. spring 0 0 summer 6 23.5 fall 12 0 winter 18 -23.5

Equatorial coords.: 

Equatorial coords.

Basic motions: 

Basic motions Our Earth is rotating. ~24hrs Our Earth is revolving. ~1year Actual motion VS apparent motion Daily motion VS annual motion

A changing night sky: 

A changing night sky A star (a planet, the moon) rises in the east and sets in the west. Its daily path is a circle parallel to the celestial equator. Some stars never rise. (far below your horizon) Some star never set. (circumpolar) During different time of a year, the Sun is blocking different stars or constellations.

The Sun’s path: 

The Sun’s path Daily path: a circle parallel to celestial equator Annual path: ecliptic

The Sun’s path: 

The Sun’s path Show demo: Masteringastronomy fig. 2.12, 2.14, 2.17

RA: 

RA Reference date: RA of the star on the meridian (transit) at midnight Sep. 21 ===> 0 hr Dec. 21 ===> 6 hr Mar. 21 ===> 12 hr June 21 ===> 18 hr RA of the Sun on the meridian (at noon, off course) Sep. 21 ===> 12 hr Dec. 21 ===> 18 hr Mar. 21 ===> 0 hr June 21 ===> 6 hr

RA: 

RA During one night, larger RA transits later. During one year, everything transits 2 hours earlier after one month. Two kinds of questions: What RA is on the meridian at a given date and time? On what day does a given RA transit at a specific time?

Example: 

Example What’s the RA of the star that transits at midnight on Sep. 6th? Sep. 21 ===> 0 hr Dec. 21 ===> 6 hr Mar. 21 ===> 12 hr June 21 ===> 18 hr 1.choose Sep. 21st as the reference date 2.6th is half month earlier than 21st, that’s 1hr difference 3.0 hr-1 hr=24 hr-1 hr =23 hr

Example: 

Example What’s the RA of the star that transits at 10pm on Sep. 5th? Lager RA transits later. 10pm is two hours before midnight. So RA should be smaller. 23hr-2hr=21hr What’s the RA of the star that transits at 3am on Sep. 6th? three hours later after midnight. 23hr+3hr=2hr

Example: 

Example Where is the star with RA=23hr on Sep. 21st at midnight? half a month later, so it should transit 1hr earlier. That’s 11pm. It already transits. It’s to the west of your meridian

Example: 

Example A constellation is at RA = 14 hr. On what day will it cross the meridian at 9:00 PM? When 14 hr crosses the meridian at 9:00 PM, 17 hr crosses the meridian at midnight Sep. 21 ===> 0 hr Dec. 21 ===> 6 hr Mar. 21 ===> 12 hr June 21 ===> 18 hr 1 hour difference between 18 hr and 17 hr. half a month before June 21. That’s June 6th.