Telling Time by the Sun Martha P. Haynes Goldwin Smith Professor of Astronomy Cornell University CAU Study Tour June 2014 The Arecibo Legacy Fast ALFA (ALFALFA) Survey http://stefanoderosa.com
ALFA is not a car… It is a radio “camera” Arecibo L-band Feed Array (ALFA)!
The ALFALFA Census of Gas- Bearing Galaxies at z ~ 0 Arecibo Legacy Fast ALFA (ALFALFA) Survey
The sky here this week Day sunrise sunset moonrise moonset moonphase Mon 05h32m 21h07m 23h38m 09h24 Waning gibbous 16 0.85 full Tue 05h32m 21h07m 00h14m* 10h38 Waning gibbous 17 0.75 full Wed 05h32m 21h07m 00h46m* 11h51m Waning gibbous 18 0.65 full Thu 05h29m 21h02m 01h14m* 12h57m Last quarter 19 Fri 05h30m 21h03m 01h45m* 14h06m Waning crescent 20 0.43 Sat 05h30m 21h03m 02h18m* 15h14m Waning crescent 21 0.32 http://aa.usno.navy.mil/data/docs/RS_OneDay.php Summer solstice Parma: 10° 20’ E, 44 ° 48’ N Bologna: 11 ° 21’ E, 44 ° 30’ Sat June 21 @ * On following day 10h51m UT = 12h51m CEST
For later in the week: Bologna on Saturday http://aa.usno.navy.mil/data/docs/RS_OneDay.php Bologna: 11° 21’ E, 44 °30 ’ Summer solstice Sat June 21 @ 10h51m UT = 12h51m CEST
What the ancients observed. 1. The Earth is spherical. 2. The Sun rises in the East and sets in the West every ~24 hours (diurnal westward motion). 3. Over the course of a year, the position of the Sun drifts slowly eastward among the stars and returns to the same place among the stars every ~365 days (annual eastward drift). 4. The Sun, Moon and planets are confined to a band stretching around the sky (ecliptic = zodiac). 5. The phases of the Moon; times of Moon Rise, Moon set. 6. Eclipses of the Sun and of the Moon 7. Differences in the motions of the planets .
What we already know that the ancients didn’t. 1. The Earth rotates on its axis once every ~24 hours (day). 2. The Earth revolves around the Sun once every ~365 days (year). 3. The Moon revolves around the Earth once every ~28 days (month). 4. The orbital planes of objects in the Solar System lie (almost) in the equatorial planes of the major body (but not quite; there are a few exceptions) 5. The Earth’s spin axis is tilted by 23½° with respect to its orbital plane around the Sun (ecliptic).
The Constellations Stars that appear close in the sky may not actually be close in space:
The Earth’s Orbital Motion • The path of the Sun’s motion through the constellations of stars is called the ecliptic. • The 12 constellations that Sun moves through during the year are called the constellations of the zodiac.
The Earth’s orbital motion • Seasonal changes in the constellations visible in the night sky are due to Earth’s motion around Sun
Celestial Sphere Celestial sphere: Imaginary sphere of huge radius centered on Earth and aligned with Earth’s poles. The concept of the celestial sphere provides a convenient visual framework for understanding the apparent motions of the Sun, Moon, planets and stars. “Coordinates”: Right ascension (like longitude) Declination (like latitude)
Three Perspectives of the Sky Heliocentric Geocentric Local Centered on Sun with Centered on Earth Centered on observer Sun fixed with (Your view of the sky) Earth fixed (not moving) Celestial sphere: Imaginary sphere of huge radius centered on Earth and aligned with Earth’s poles. Our view of the celestial sphere depends on our location on Earth.
Local Perspective Follow along with Zenith Local the practice meridian page handout Local west Local Local south north Local east Horizon The azimuth is the angular distance along the horizon from the north point measured in the eastward direction.
Local Perspective Zenith Local meridian W Az= 270º Az= S N Az= 0º 180º Az= 90º E The azimuth is the angular distance along the horizon from the north point measured in the eastward direction.
Local Perspective Zenith Local NCP: meridian North Celestial Pole W altitude N S E Horizon The altitude of the North Celestial pole (as measured up from the horizon) is equal to the latitude of the observer’s location
Local Perspective C.Eq. NCP: W altitude N S The celestial equator crosses the meridian 90° to the south (measured along the meridian) of the NCP.
Local Perspective C.Eq. NCP W altitude N S The altitude of point where the celestial equator crosses the local meridian is 180º - altitude of NCP - 90º = altitude of C.Eq. = 90º - altitude of NCP
Diurnal Motion • As the Earth rotates, a star (the Sun, the Moon or a planet), appears to move across the sky, from East to West, following a path parallel to the Celestial Equator. Along Lines of Constant Diurnal East to Declination, parallel to Motion West the Celestial Equator • Since the orientation of the lines of Declination depend on the location (latitude) on Earth, the path a star (the Sun, the Moon or a planet) takes across the sky depends on the latitude of the observer’s location on Earth.
The zodiac = the ecliptic During the year, the Sun appears to move through the constellations of the Zodiac. In astronomical lingo, the zodiac is the “ecliptic”.
The ecliptic: the annual path of the Sun Due to the Earth’s orbital revolution, the apparent position of the Sun in the sky appears to move along a path tilted by 23½ deg with respect to the celestial Equator (higher up North in northern Summer, higher up South in southern Summer. The apparent path of the Sun amid the stars in the sky is referred to as the “ ecliptic ”.
The Sun’s Apparent Path • The Sun’s apparent position among the stars changes throughout the year. Eastward annual drift • Unlike a star, the Sun (Moon and planets) moves with respect to the (much more distant) stars. • Right Ascension and Declination of the Sun (Moon and planets) change throughout the year . • The path the Sun (Moon and planets) takes across the sky on any given day depends on its Declination on that day. • Noon-time altitude (above horizon) varies • Length of time to cross from East to West along the path on a given day varies = length of daylight.
The Sun’s Apparent Path • The Sun’s apparent position among the stars changes throughout the year. Eastward annual drift • Unlike a star, the Sun (Moon and planets) moves with respect to the (much more distant) stars. • Right Ascension and Declination of the Sun (Moon and planets) change throughout the year . • The path the Sun (Moon and planets) takes across the sky on any given day depends on its Declination on that day. • Noon-time altitude (above horizon) varies • Length of time to cross from East to West along the path on a given day varies = length of daylight. The Earth experiences seasons!
Tilt of the Earth on its axis
The Ecliptic Plane
The Sun’s Path Throughout the Year • The Sun’s Declination changes throughout the year due to the inclination of the Earth on its axis. • On Sep 20 th and Mar 20 th , the Sun’s Declination is 0°. • The Sun’s path follows the Celestial Equator. • These are called the autumnal and vernal equinoxes. • On Dec 21 st , the Sun’s Declination is -23½°. • At noon, the Sun crosses the meridian south of the Celestial Equator by 23½°. • Length of daylight shortest in North; longest in South. • Winter in the northern hemisphere; summer in the South. • On Jun 21 st , the Sun’s Declination is +23½ °. • At noon, the Sun crosses the meridian north of the Celestial Equator by 23½°. • Length of daylight longest in North; shortest in South • Summer in the northern hemisphere; winter in the South.
The Sun’s Path Throughout the Year Facing south
The Sundial in Busseto • Busseto is a commune in the province of Parma with a population of ~ 7100 • Rocca built in 1 st half of 13C; destroyed and rebuilt in 1857. • Giuseppe Verdi was born in Roncole in 1813 and moved to “town” ( Busseto) in 1824.
Basic scheme of a vertical sundial
The Sundial in Busseto
A not quite south-facing sundial
Zodiac symbols
The sky as seen from Busseto
The sky as seen from Busseto
The sundial in Busseto
The Sun today in Busseto Busseto 44° 59’ N, 10 ° 02’ E (44.98333,10.03333)
The Sun today in Busseto Busseto 44° 59’ N, 10 ° 02’ E (44.98333,10.03333)
South-facing vertical sundial E W S
South-facing vertical sundial E W S
Sundial in Busseto
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