Patterns in Celestial Motion By Dr. Harold Williams of Montgomery College Planetarium

Patterns in Celestial Motion

By Dr. Harold Williams

of Montgomery College Planetariumhttp://montgomerycollege.edu/Departments/planet/

Time counting repeating patternsin the sky

• A second, 1/60 of a minute, 1/(24*60*60)=1/86,400 of a day• A minute, 1/60 of a hour, 1/(24*60)=1/1440 of a day• A hour, 1/24 of a day• DAY, originally from sun rise to sun rise or sun down to sun down,

now a rotation of the earth about its axis• A week, Jewish bundling of days, sacred calendars• MONTH: originally a lunation (same moon to identical moon)• YEAR: originally a revolution of the sun around the earth through the

ecliptic, now a repeat of the earth around sun in its orbit. • A decade, century, millennium: different bundling of years• PRECESSION CYCLE: originally motion of the first day of a season

around the ecliptic, now cycle of the rotation axis (day and night axis) about the year axis (this angle is around 23 ½ degrees now and takes around 26,000 years)

• GALACTIC YEAR: revolution of solar system about the Galactic center in an orbit of around 250 million years.

Day and Night most basic, what you see every day, without going into

outer space!

From outer space since ~300BCE

First of all earth is round

• It looks flat locally to me and you.• Aristotle, shadow of the earth on the moon

during a lunar eclipse is always round.• Greek sailors observed that when ships left

the harbor the top of the mast disappeared last and the bottom of the ship first. So the earth is definitely curved.

• But does the central fire, the sun, move around the earth or the earth rotate.

History and Social Studies is very important in Science (Aerospace)

• Evolution of models to explain things over time.• You really can’t understand day and night motion

without folding in yearly motion particularly when you start observing the changes in the night sky through out the year. This started a long time ago.

• Language, reading and writing are essential.• Mathematics, the self consistent abstraction of

reality is very useful.

Celestial Sphere

Celestial sphere on Latitude Protractor

Celestial Sphere Rectified forthe North Pole

Celestial Sphere Rectified forthe Equator

Celestial Sphere Rectified forthe some place like here

The Celestial Sphere• Zenith = Point on the

celestial sphere directly overhead

• Nadir = Point on the c.s. directly underneath (not visible!)

• Celestial equator = projection of Earth’s equator onto the c.s.

• North celestial pole = projection of Earth’s north pole onto the c.s.

The Celestial Sphere (II)• From geographic

latitude ℓ (northern hemisphere), you see the celestial north pole ℓ degrees above the horizon;

• From geographic latitude - ℓ (southern hemisphere), you see the celestial south pole ℓ degrees above the horizon.

• Celestial equator culminates 90o – ℓ above the horizon.

ℓ

90o - ℓ

Night Time Star Motion

Round Earth Rotating now

Putting DAY and YEARtogether the modern way

The Annual Motion of the sun

Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

The sun’s apparent path on the sky is called the ecliptic.

Equivalent: The ecliptic is the projection of Earth’s orbit onto the celestial sphere.

The Seasons (IV)

sun

Earth in JulyEarth in January

Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.

Earth’s orbit (eccentricity greatly exaggerated)

The Motion of the Planets (I)

The planets are orbiting the sun almost exactly in the plane of the ecliptic.

The moon is orbiting Earth in almost the same plane (ecliptic).

Jupiter

MarsEarth

Venus

Mercury

Saturn

Precession