Telling Time by the Sun Martha P. Haynes Goldwin Smith Professor of - - PowerPoint PPT Presentation

Telling Time by the Sun

Martha P. Haynes

Goldwin Smith Professor of Astronomy

Cornell University CAU Study Tour June 2014

http://stefanoderosa.com The Arecibo Legacy Fast ALFA (ALFALFA) Survey

ALFA is not a car…

Arecibo L-band Feed Array (ALFA)!

It is a radio “camera”

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 16 05h32m 21h07m 23h38m 09h24 Waning gibbous 0.85 full Tue 17 05h32m 21h07m 00h14m* 10h38 Waning gibbous 0.75 full Wed 18 05h32m 21h07m 00h46m* 11h51m Waning gibbous 0.65 full Thu 19 05h29m 21h02m 01h14m* 12h57m Last quarter Fri 20 05h30m 21h03m 01h45m* 14h06m Waning crescent 0.43 Sat 21 05h30m 21h03m 02h18m* 15h14m Waning crescent 0.32

http://aa.usno.navy.mil/data/docs/RS_OneDay.php Parma: 10°20’ E, 44°48’ N Bologna: 11°21’ E, 44°30’ * On following day Summer solstice Sat June 21 @ 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
• rbital 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 Sun fixed Centered on Earth with Earth fixed (not moving) Centered on observer (Your view of the sky) 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

Zenith Local north Local south Local meridian The azimuth is the angular distance along the horizon from the north point measured in the eastward direction. Local east Local west Horizon

Follow along with the practice page handout

Local Perspective

Zenith Local meridian The azimuth is the angular distance along the horizon from the north point measured in the eastward direction. W E S N Az= 0º Az= 180º Az= 90º Az= 270º

Local Perspective

Zenith N S NCP: North Celestial Pole Local meridian The altitude of the North Celestial pole (as measured up from the horizon) is equal to the latitude of the observer’s location altitude Horizon W E

Local Perspective

N S NCP: C.Eq. altitude The celestial equator crosses the meridian 90° to the south (measured along the meridian) of the NCP. W

Local Perspective

N S NCP C.Eq. altitude W 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.

Diurnal Motion East to West

Along Lines of Constant Declination, parallel to 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

• r 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”.

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 ecliptic: the annual path of the Sun

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 20th and Mar 20th, the Sun’s Declination is 0°.
• The Sun’s path follows the Celestial Equator.
• These are called the autumnal and vernal equinoxes.
• On Dec 21st, 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 21st, 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 1st 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

S W E

South-facing vertical sundial

S W E

Sundial in Busseto