Time, coordinates and how the Sun and Moon

move in the sky

• Using the colors and magnitudes of quasars drawn from the SDSS Catalog Archive Server to distinguish quasars from stars using the light they emit

• Studying the motions of the Sun and the Moon in the night sky in order to predict eclipse times

• We’ll need to know about

• times and time-zones

• East-West and North-South coordinates

• How the Sun and Moon appear to move East-West and North-South over a day, a month, a year etc.

Last Time...

This Time...

• Given different time systems, leap years etc. it is useful to have a calendar with which to express exact times of events in the sky (referred to as epochs)

• In astronomy we use a calendar based on the original Julian calendar (established by Julius Caesar)

• Julian Date (JD) is a count forward in days staring from 0 at noon on January the 1st in the year -4712 (4713 BCE)

• Modified Julian Date (MJD) is a count in days from 0 midnight on November 17 in the year 1858

• MJD = JD - 2400000.5

• The YYYY-MM-DD date you’re used to is called the ISO date (it’s set by the International Standardization Organization)

Precise timekeeping and MJD

Prime MeridianGreenwich Mean Time (GMT)MST (MDT is 1 hour

closer to GMT)

Time zones

• Any position on the surface of a sphere (such as the Earth or the night sky) can be expressed in terms of the angular coordinateslatitude and longitude

• Latitude runs from -90o to 90o

• -90o is a sphere’s south pole (South Pole on Earth, South Celestial Pole in the sky)

• 90o is a sphere’s north pole (North Pole on Earth, North Celestial Pole in the sky)

• 0o is a sphere’s equator (the Equator on Earth, the Celestial Equator in the sky)

Coordinates on the Spherelatitude

(δ)

longitude (α)

Equator

• Longitude runs from 0o to 360o (it’s chosen to instead be -180o to 180o on the Earth)

• In this equatorial coordinate system used in astronomy

• longitude is called RA orRight Ascension

• and latitude is called Dec orDeclination

• We will use these coordinates. If you like, think of them as simply a pair of coordinates that specify a point in the sky

• We will always express these coordinates in degrees ... note that an arcminute is one-sixtieth (1/60) of a degree

The Equatorial Coordinate System

latitude(δ)

longitude (α)

Equator

• Right Ascension and Declination are measured relative to a fixed system of very distant so motionless stars

• The Earth spins and orbits relative to an axis along it’s North to South Pole

• Because of this, the Sun’s Right Ascension (longitude) and Declination (latitude) are affected by different things

• The Sun’s RA (East-West position compared to distant, fixed background stars) is affected by East-West motions

• The Earth orbiting the Sun

• The Sun’s Dec is affected by North-South motions• Unless you walk north or south around the Earth, the

only external effect on the Sun’s Dec is the seasons

North-South & East-West Motions

Sun’s East-West Motion…looking down on the Earth’s North Pole

~September 21

~March 21

This star is visible at night

The other star is up during the

day and hiddenby the Sun

This star is visible at night

The other star is up during the

day and hiddenby the Sun

The position of the Sun compared to the fixed stars in the sky moves 360o per year

Looking down on Earth’s North Pole, Earth’s rotation and orbit are counter-clockwise

RA=0oRA=180o RA=180oRA=0o

What Causes the Seasons?• It’s the tilt of the Earth’s rotation axis (North-South

Pole line) to the Earth’s orbital plane• the Earth’s orbital plane is called the ecliptic

• Note, the tilt direction doesn’t change over the year

Approximate Solstices & Equinoxes in the Earth’s Northern Hemisphere• Winter Solstice – December 21

• night longer than day in the Northern Hemisphere• Vernal (Spring) Equinox – March 21

• 12 hours of daylight, 12 hours of night• Summer Solstice – June 21

• day longer than night in the Northern Hemisphere• Autumnal (Fall) Equinox – September 21

• 12 hours of daylight, 12 hours of night

Sun’s East-West Motion: Thoughts• The tilt of the Earth to its orbital plane is ~23.5o

(this is formally called Earth’s axial tilt)• The equator is at 0o latitude on Earth (which is

the same thing as 0o declination in the sky)• So, the Sun moves from +23.5o declination

(23.5oN, the Summer Solstice in the Northern Hemisphere) to -23.5o (23.5oS, the Winter Solstice in the Northern Hemisphere)

• Casper is near 43o geographic latitude, which is the same thing as a line of 43o declination in the sky

Reminder: Basics of Eclipses• Lunar Eclipse

• When the Moon passes into the Earth's shadow

• Sun – Earth – MoonFull Moon

• Solar Eclipse• When the Earth crosses

the Moon's shadow• Sun – Moon – Earth

New Moon• Why don’t eclipses happen

every Full and New Moon?

The Moon’s Motion Relative to the Sun

• The Moon’s orbit is tilted by 5º to the Earth’s orbital plane (which, remember is called the ecliptic)

• The Moon appears to be about ½o in angular size in the sky, so 5o is ~10 Moon diameters

• The Moon must also be near the ecliptic for an eclipse to occur

(once each month!)

Time, coordinates and how the Sun and Moon

move in the sky