Chapter 2 - Understanding the Sky

The Moon

- Maria regions are generally named in Latin. Galileo originally thought these regions were liquid seas (Mare is latin for sea) and astronomers have retained that convention in their naming. - Impact craters are generally given the names of famous astronomers. • Two sides of the moon - A thinner crust on the "light" side leads to more mariaand fewer highlands

Summary of eclipes: Two conditions must be met to have an eclipse:

1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

The Greeks knew that the lack of observable parallax could mean one of two things:

1. Stars are so far away that stellar parallax is too small to notice with the naked eye. 2. Earth does not orbit the Sun; it is the center of the universe. With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away. - Thus, the stage was set for the long, historical showdown between Earth-centered and Sun-centered systems.

What is the age of the Universe?

14 billion years

Altitude of the celestial pole = your latitude

Altitude, sometimes referred to as elevation, is the angle between the object and the observer's local horizon. For visible objects it is an angle between 0 degrees to 90 degrees.

What is the arrow pointing to in the photo below? A. the zenith B. the north celestial pole C. the celestial equator

B. the north celestial pole

The angular size of your finger at arm's length is about 1°. How many arcseconds is this? A. 60 arcseconds B. 600 arcseconds C. 60 *60 = 3600 arcseconds

C. 60 *60 = 3600 arcseconds

The North Star (Polaris) is 50°above your horizon, due north. Where are you? A. You are on the equator. B. You are at the North Pole. C. You are at latitude 50°N. D. You are at longitude 50°E. E. You are at latitude 50°N and longitude 50°E.

C. You are at latitude 50°N.

The brightest stars in a constellation A. all belong to the same star cluster. B. all lie at about the same distance from Earth. C. may actually be quite far away from each other.

C. may actually be quite far away from each other.

It's 9 a.m. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? A. first quarter B. waxing gibbous C. third quarter D. half moon

C. third quarter

TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.

FALSE • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. • The real reason for seasons involves Earth's axis tilt.

If the Sun rises precisely due east and you don't live on the equator:

It must be the day of either the spring or fall equinox.

Another thought:

Near the poles -Polar day and polar night

Which of these is a constellation?

Orion

If there is going to be a total lunar eclipse tonight, then you know that:

The Moon's phase is full.

Phases of the Moon: 29.5-day cycle

Waxing • Moon visible in afternoon/evening • Gets "fuller" and rises later each day Waning • Moon visible in late night/morning • Gets "less full" and sets later each day

The North Celestial Pole is 45.0° above your northern horizon. This tells you that:

You are at latitude 45.0° N.

Constellations

• A constellation is a region of the sky. - This term is also used to describe a pattern of stars • Eighty-eight constellations fill the entire sky.

How does the orientation of Earth's axis change with time?

• Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. - Polaris won't always be the North Star. - Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces

Angular Size

• An object's angular size appears smaller if it is farther away. angular size = physical size * (360 degrees/(2 pi * distance))

The sky varies as Earth orbits the Sun

• As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. • At midnight, the stars on our meridian are opposite the Sun in the sky.

Lunar Phases

• Both the near and far sides of the moon go through periods of light and dark • We can see this in terms of lunar phases (new, quarter, full, crescent, etc...) - Whatever fraction of our side of the moon is dark, that percentage of the far side is bright - (half the moon is always illuminated

Why The Arc?

• Celestial Sphere and its grid • This means all objects will appear to move in parallel circles across the sky, according to their declinations - These declination lines are called "Diurnal Circles"

Coordinates systems of the sky:Horizon (Horizontal) Coordinate System

• Divides the sky into the upper hemisphere (objects are visible) and the lower hemisphere (view of objects blocked by Earth) • Celestial horizon (the great circle on the celestial sphere) separates the hemispheres • Two independent angular coordinates (altitude and azimuth) - Altitude (Alt), sometimes referred to as elevation, is the angle between the object and the observer's local horizon. For visible objects, it is an angle between 0 degrees to 90 degrees. - Alternatively, zenith distance may be used instead of altitude. The zenith distance is the complement of altitude so that the sum of the altitude and the zenith distance is 90° • Two independent angular coordinates (altitude and azimuth) - Azimuth (Az) is the angle of the object around the horizon, usually measured from the northincreasing towards the east(some exceptions exist)

Why do stars rise and set?

• Earth rotates from west to east, so stars appear to circle from east to west. • We can picture this movement by imagining the celestial sphere rotating around the Earth

Explaining Apparent Retrograde Motion

• Easy for us to explain: occurs when we "lap" another planet (or when Mercury or Venus laps us). • But very difficult to explain if you think that Earth is the center of the universe! • In fact, ancients considered but rejected the correct explanation.

We measure the sky using angles.

• Full circle = 360º • 1º = 60' (arcminutes) • 1'= 60'' (arcseconds)

The Celestial Sphere

• Imaginary sphere surrounds the Earth (as viewed from the Earth) • Stars at different distances all appear to lie in the celestial sphere. • The 88 official constellations cover the celestial sphere. • The Ecliptic is the Sun's apparent path through the celestial sphere. • North celestial pole is directly above Earth's North Pole. • South celestial pole is directly above Earth's South Pole. • Celestial equator is a projection of Earth's equator onto sky.

Coordinates on the Earth

• Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) - The sky varies with latitude but not with longitude

The Local Sky

• Looks like a dome because we see only half the celestial sphere • It's useful to have some reference points: zenith, meridian, horizon • We can locate any object by specifying its altitude above the horizon and direction along the horizon. • An object's altitude (above horizon) and direction (along horizon) specify its location in your local sky • Meridian: line passing through zenith and connecting N and S points on horizon • Horizon: all points 90° away from zenith • Zenith: the point directly overhead

Lunar eclipse

• Lunar eclipses can occur only at full moon. • Lunar eclipses can be penumbral, partial, or total. • In a lunar eclipse,the Moon passes completely into the shadow of Earth.

Phases of the Moon

• Lunar phases are a consequence of the Moon's 27.3-day orbit around Earth. • Half of Moon is illuminated by Sun and half is dark. • We see a changing combination of the bright and dark faces as Moon orbits.

Coordinates systems of the sky: Equatorial Coordinate System

• Measured with respect to distant stars • The coordinates are based on the location of stars relative to Earth's equator if it were projected out to an infinite distance Describes the sky as seen from the solar system, and modern star maps almost exclusively use equatorial coordinates. • Two independent angular coordinates (right ascension and declination) - Right ascension (RA) (aka hour angle) is the celestial equivalent of longitude. Measured from a primary direction (vernal equinox), which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north - RA is measured continuously in a full circle from vernal equinox towards the east - Customarily measured in hours (h), minutes (m), and seconds (s), with 24hbeing equivalent to a full circle (because a star's location is measured by timing its passage through the highest point in the sky as the Earth rotates) • Two independent angular coordinates (right ascension and declination) - Declination (dec) is comparable to geographic latitude projected onto the celestial sphere - North of the celestial equator = positive dec; south = negative dec - Customarily measured in degr, min and sec - Dec > 90⁰ does not occur

Planets Known in Ancient Times

• Mercury - difficult to see; always close to Sun in sky • Venus - very bright when visible; morning or evening "star" • Mars - noticeably red • Jupiter - very bright • Saturn - moderately bright

Parallax

• Parallax is the apparent shift in position of a nearby object against a background of more distant objects.

Seasonal changes are more extreme at high latitudes.

• Path of the Sun on the summer solstice at the Arctic Circle

What was once so mysterious about planetary motion in our sky?

• Planets usually move slightly eastward from night to night relative to the stars. • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion. - We see apparent retrograde motion when we pass by a planet in its orbit.

Rotation

• Rise in the East, sets in the West. • The Earth spins once every 24 hours • As we rotate West to East, we see the sky "rotate" from East to West

What causes the seasons?

• Seasons depend on how Earth's axis affects the directness of sunlight. • Axis tilt changes directness of sunlight during the year • Direct light causes more heating - Notice the axial tilt: 23.5°

Solar Eclipse

• Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, or annular. • In a solar eclipse,because of the large size of the Sun, the rays taper to provide an umbra (total eclipse) and a surrounding penumbra (partial eclipse).

Our view from Earth:

• Stars near the north celestial pole are circumpolar and never set. • We cannot see stars near the south celestial pole. • All other stars (and Sun, Moon, planets) rise in east and set in west.

We can recognize solstices and equinoxes by Sun's path across sky:

• Summer (June) solstice: highest path; rise and set at most extreme north of due east • Winter (December) solstice: lowest path; rise and set at most extreme south of due east • Equinoxes: Sun rises precisely due east and sets precisely due west.

Sun's altitude also changes with seasons.

• Sun's position at noon in summer: Higher altitude means more direct sunlight. • Sun's position at noon in winter: Lower altitude means less direct sunlight.

We see only one side of Moon

• Synchronous rotation: the Moon rotates exactly once with each orbit. • That is why only one side is visible from Earth.

Polaris

• Ternary star system (3 stars) • In constellation Ursa Minor (Little Bear) • ~434 light-years away • "The North Star" - It's always due North

What causes eclipses?

• The Earth and Moon cast shadows. • When either passes through the other's shadow, we have an eclipse.

Why don't we have an eclipse at every new and full moon?

• The Moon's orbit is tilted 5°to ecliptic plane. • So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.

Arcs

• The arc-like path of the diurnal circles is responsible for the rising and setting of the sun, moon, planets, and stars - There's a special star that spends varying amounts of time visible to us • The Sun spends different amounts of time above the horizon throughout the year (days are much longer in the summer time) • The arc-like path of the diurnal circles is responsible for the rising and setting of the sun, moon, planets, and stars • But...what determines how long something is visible (above the horizon)? - Three categories • circumpolar (never set) • sub-horizonal(never rise) • different length of time in the sky

Why did the ancient Greeks reject the real explanation for planetary motion?

• Their inability to observe stellar parallax was a major factor.

Why do the constellations we see depend on latitude and time of year?

• They depend on latitude because your position on Earth determines which constellations remain below the horizon. • They depend on time of year because Earth's orbit changes the apparent location of the Sun among the stars.

Why doesn't distance matter?

• Variation of Earth-Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt. • Variation in any season of each hemisphere-Sun distance is even smaller!

How do we mark the progression of the seasons?

• We define four special points: summer (June) solstice winter (December) solstice spring (March) equinox fall (September) equinox

Patterns of motion

• When we watch the sky over a period of time what do we see? MOTION • "Rise in the East, Set in the West" - Everything in the sky (that we didn't put up there ourselves) moves in this fashion: stars, planets, the sun, the moon... - What does it look like exactly?

North Celestial Pole

• Where is the North Celestial Pole? - Top of the planet. Straight up in space. • Why is it called the North Celestial Pole? - It's like the north pole, but now in space. - Just like the Earth rotates around the north and south poles, if you extend these lines out into space, the Earth rotates around that line too. • Back to the rotation of the Earth! • Why North Celestial Pole? -- There's a super convenient star there!

Reason for the Seasons!

• Why does the Earth have seasons? • A common misconception is that the Earth is closer to the Sun in summer than in winter, just like it feels warmer when you are closer to a campfire. • But... it turns out that it's exactly opposite......and the main reason for the seasons is...the Earth's tilt!

What does the universe look like from Earth?

• With the naked eye, we can see more than 2000 stars as well as the Milky Way. • A band of light making a circle around the celestial sphere. - What is it? • Our view into the plane of our galaxy.

Dividing the sky: Asterisms and constellations

•- Two categories • Asterisms • Constellations •- Asterisms are completely arbitrary: • Draw a picture with the stars • Connect the dots in your head •- Constellations are much stricter: • Defined as a region of space • All stars contained in this space (visible or not) are part of the constellation

Stellar parallax

•Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun. • Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star. • Parallax angle depends on distance.