astronomy 161depoy/astro161/notes/class2.pdf · the celestial sphere: key concepts (1) the sky as...

ASTRONOMY 161Introduction to Solar System Astronomy

Astronomy 161: The Web Pagewww.astronomy.ohio-state.edu/~depoy/Astro161/astro161.html

Astronomy Picture of the Day (APOD):

http://antwrp.gsfc.nasa.gov/apod/

The Celestial SphereFriday, January 5

The Celestial Sphere: Key Concepts(1) The sky as seen from Earth is divided into 88

constellations.(2) It is convenient to pretend the stars are attached to a

celestial sphere.(3) The celestial sphere appears to rotate about the celestial

poles (1 day).

(4) The Sun appears to move west to east relative to stars(1 year).

(5) The Moon appears to move west to east relative to stars(1 month).

(1) The sky is divided into 88 constellations

Mesopotamia, circa 3000 BC:oldest know constellations

Ptolemy, 2nd century AD:48 constellations in northern sky

16th to 18th century AD:unmapped regions of sky filled in

Constellations are largely arbitrary

Other cultures, other constellations:

Example: Ursa Major = bear, dipper, bull's leg,grain scoop, wagon, plow, etc.

Stars in a constellation usually are not at the samedistance from us.

At a different place in our Galaxy, we would seedifferent star patterns.

A modern star chart of Ursa Major:

(2) Stars are “attached” to a celestial sphere

Distances to stars are hard to measure.

However, we can pretend all stars are at thesame distance from us, attached to a largecelestial sphere.

Position on the celestial sphere is known evenwhen the distance in unknown.

Celestial Sphere: A large imaginary sphere centered on Earth

Special locations on the celestial sphere

North Celestial Pole = point directly above Earth’sNorth Pole (near the star Polaris)

South Celestial Pole = point directly above Earth’sSouth Pole (no nearby bright star)

Celestial Equator = circle directly above Earth’sEquator

Distances between points on thecelestial sphere are measured in

degrees, arcminutes, and arcseconds

360 degrees in a circle

60 arcminutes in a degree

60 arcseconds in an arcminute

½ degree = angular size of

Sun & Moon

Celestial navigation made simple

At Earth’s North Pole:Polaris is directly overhead

At Earth’s Equator:Polaris is due north, on the horizon

In Earth’s Northern hemisphere:Polaris is due north - height above the horizon(in degrees) is equal to your latitude (in degrees)

(3) The celestial sphere appears to rotate aboutthe celestial poles (1 day cycle)

Observation: Stars, Sun, Moon and planetsmove in counterclockwise circles aroundnorth (south) celestial pole.

Objects near the celestial equator move east towest when above the horizon (“rising” ineast, “setting” in west).

What causes these circular motions?

Explanations of the 1 day cycle:

HYPOTHESIS #1(Ptolemy, 2nd century):Earth is stationary; stars

are attached to a spherethat revolves around

the Earth once per day.

WRONG!

HYPOTHESIS #2(Copernicus, 16th cent.):

Stars are stationary; Earthrotates about its axis

once per day.

RIGHT!

Nicolaus Copernicus (1473-1543)

(4) The Sun appears to move west to eastrelative to stars (1 year cycle)

Today the Sun is “in” Virgo, next month inLibra, etc.

Sun’s path on the celestial sphere = ecliptic

Constellations through which the ecliptic runs= zodiac

The ecliptic is NOT the same as the celestialequator!

Observation: Sun moves west to east relativeto stars (about 1 degree per day).

What causes this annual motion?

Explanations of 1 year cycle:

HYPOTHESIS #1(Ptolemy):

Sun revolves aroundEarth at a slightly

slower rate than thecelestial sphere.

WRONG!

HYPOTHESIS #2(Copernicus):

Earth revolves around theSun, once per year.

RIGHT!

(5) The Moon appears to move west to eastrelative to stars (1 month cycle)

Today the Moon is “in” Virgo

In two weeks: Pisces

In four weeks: Virgo, again.

Observation: Moon moves west to east relative tostars, taking 27.3 days to complete cycle.

What causes this monthly motion?

Explanations of 1 month cycle:

HYPOTHESIS #1(Ptolemy):

Moon revolves aroundEarth at a significantly

slower rate than thecelestial sphere.

WRONG!

HYPOTHESIS #2(Copernicus):

Moon revolves aroundEarth, once per month.

RIGHT!