Last time: 2.1. Patterns in The Sky: Stars and constellations, TODAY:

celestial coordinates2.2 Seasons,…Precession2.2 (cont.). 2.3. The Moon and Eclipses

Lunar Phases and Eclipses

2.4 Ancient Mystery of the Planets: Apparent Retrograde motion of

planets Parallax

Ch 3 (Histrory of Astronomy)

Class 3: End of Ch 2 and Ch. 3

The Celestial SphereNorth & South celestial poles

the points in the sky directly above the Earth’s North and South poles

celestial equatorthe extension of the Earth’s equator onto the celestial sphere

eclipticthe annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane

The Celestial Sphere

The Milky Way

You’ve probably seen this band of light across the sky.What are we actually seeing?

The Milky Way•Our Galaxy is shaped like a disk.•Our solar system is in that disk.•When we look at the Milky Way in the sky, we are looking along that disk.

Measuring the Sky

We measure the sky in angles, not distances.

• Full circle = 360º

• 1º = 60 arcmin

• 1 arcmin = 60 arcsec

Measuring Angles in the Sky

The Local Sky

zeniththe point directly above you

horizonall points 90° from the zenith

altitudethe angle above the horizon

meridiandue north horizon zenith due south horizon

Review: Coordinates on the Earth• Latitude: position north or south of equator

• Longitude: position east or west of prime meridian (runs through Greenwich, England)

Review: Coordinates on the Earth• Latitude: position north or south of equator

• Longitude: position east or west of prime meridian (runs through Greenwich, England)

In Celestial sphere the equivalent coordinate are• Declination: (more later)• Right Ascension: (more later)

The Daily Motion

• As the Earth rotates, the sky appears to us to rotate in the opposite direction.

• The sky appears to rotate around the N (or S) celestial poles.

• If you are standing at the poles, nothing rises or sets.

• If you are standing at the equator, everything rises & sets 90 to the horizon.

Time Exposure Photograph:

Star Trails

The Daily Motion

daily circles --- CCW looking north, CW looking south

Annual Motion

• As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars.

• The Sun circles the celestial sphere once every year.

2.3 Seasons

• What is the cause of the seasons on Earth?

Annual Motion• The Earth’s axis is tilted 23.5° from being

perpendicular to the ecliptic plane.

• Therefore, the celestial equator is tilted 23.5° to the ecliptic.

• As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator.

• Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

Annual Motion

eclipticthe apparent path of the Sun through the sky

(also the plane of Earth’s orbit)equinox

where the ecliptic (the Sun) intersects the celestial equator

solsticewhere the ecliptic (the Sun) is farthest from the celestial equator

zodiac

the constellations which lie along the ecliptic

The Cause of the Seasons

Coordinates on the Celestial Sphere (not in book)

• Latitude: position north or south of equator

• Longitude: position east or west of prime meridian (runs through Greenwich, England)

• Declination: position north or south of celestial equator (in degrees)

• Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox (where the sun crosses the celestial equator going North)

What point on the celestial sphere is defined by a Right Ascension of zero hours and a Declination of zero degrees?

Question 1

What point on the celestial sphere is defined by a Right Ascension of 12 hours and a Declination of zero degrees?

Question 2

What is an equinox?

Question 3

What point on the celestial sphere is defined by being on the ecliptic at a Declination of +23.5 degrees?

Question 4

What point on the celestial sphere is defined by being on the ecliptic at a Declination of minus 23.5 degrees (i.e. 23.5 degrees South)?

Question 5

Axis tilt causes uneven heating by sunlight throughout the year.

When is summer?(in the Northern Hemisphere)

• The solstice which occurs around June 21 is considered the first day of summer.

• However, it takes time for the more direct sunlight to heat up the land and water.

• Therefore, July & August are typically hotter than June.

2.4 Precession

• What is the Earth’s cycle of precession?

Thought Question

What is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator

Thought Question

What is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator

Precession• The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years.• Precession changes the positions in the sky of the celestial poles and the equinoxes. Polaris won't always be the north star. However the tilt in the axis is the same (23.5 degrees) as the Earth’s axis precesses

A. Will Polaris be the North star in another 13,000 years?

Question

B. Will Polaris be the North star in another 26,000 years?

Question

2.3 The Moon, Our Constant Companion

• Why do we see phases of the Moon?

• What conditions are necessary for an eclipse?

Lunar Motion

Phases of the Moon’s 29.5 day cycle• new

• crescent

• first quarter

• gibbous

• full

• gibbous

• last quarter

• crescent

waxing

waning

Lunar phases and the month

• A cycle of lunar phases is approximately 30 days and is the reason why we divide the year in 12 months

• Why do lunar phases occur on different calendar dates from one year to the next?

Lunar phases and the month

• A cycle of lunar phases is approximately one month and is the reason why we divide the year in 12 months

• Why do lunar phases occur on different calendar dates from one year to the next?

• Because one year (365 days) is not exactly 12 times the period of the Moon’s phases (its actually 12.4 times).

Why do we see phases?

• Half the Moon illuminated by Sun and half dark

• We see some combination of the bright and dark faces

Moon’s position at sunset for waxing phases

(numbers are days since new Moon)

Phases of the Moon

Why do we see the same side on our Moon?

Rotation period = orbital period

Eclipses

• The Earth & Moon cast shadows.

• When either passes through the other’s shadow, we have an eclipse.

• Why don’t we have an eclipse every full & new Moon?

EclipsesWhen the Moon’s orbit intersects the ecliptic (node):

at new moon solar eclipse

at full moon

• everyone on the nighttime side of Earth can see it

lunar eclipse

you must be in Moon’s shadow to see it• within umbra: total solar eclipse• within penumbra: partial solar eclipse

Solar Eclipse

What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?

Question

What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?

Hint: where is the Sun on Sept 21?

Question

Lunar Eclipse

What have we learned?

• Why do we see phases of the Moon?• At any time, half the Moon is illuminated by the Sun

and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon.

• What conditions are necessary for an eclipse?• An eclipse can occur only when the nodes of the

Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.