Astronomy and Cosmology: Geometry & Coordinate Systems, The Earth- Sun-Moon System

Earth orbits or revolves

around the sun

Earth period of revolution

27 days

Ecliptic/ Ecliptic Plane

Earth's orbit around the sun

1 Astronomical Unit

The distance from the Earth to the Sun

(93 million miles or 8.3 light mins)

Moon orbits earth

about 60 Earth radii or about 1.3 light-seconds)

Earth rotates about an axis

that passes through the North and South

Earth around the sun rotation period

24 hours or 1 day

Axis not perpendicular

to the ecliptic plane

tilt of the Earth's axis

23.5 degrees from the perpendicular (or vertical)

How is Night and Day caused?

Rotation about Earth's axis

Rotation period of Earth's axis

24 hours

Celestial sphere

dome of the night sky rotating around us

North side of axis

points almost perfectly towards Polaris

How to find Polaris

At the tail of the little dipper

Find using Big Dipper

How fast does the night sky rotate in one full rotation of the night sky

360 degres

How fast does the night sky rotate in one day?

24 * 60 = 1440 mins

If the sky rotates 360° in 1440 mins

every minute the sky rotates 360/1440°= 0.25°per minute or a quarter degree every minute

How fast does the night sky rotate in an hour?

0.25 degrees/mins x 60 mins = 15°.

Sky rotates 15 degrees every hour.

How fast does the night sky rotate?

A quarter degree per

minute

15 degrees every

hour

If sun is 15 degrees

above horizon before

sunset, it will set in

an hour

Seasons are caused by

The tilt of the rotation axis

Depending on where the Earth is on its orbit around the Sun

one hemisphere receives more/longer

amounts sunlight during the day than the

other.

That sunlight is also less spread out.

The other hemisphere receives a

shorter amount of sunlight during the day.

The sunlight is also more spread out

because it is hitting the ground at a

shallower angle

Negligible effect caused by

the earth being so far from the sun that the slight closeness of one of the hemispheres due to the tilt causes this.

Northern Hemisphere

a small amount of sunlight spread out over the whole hemisphere

Southern Hemisphere

a lot of sunlight spread over the same area

Solstice

when the Earth's poles have the maximum tilt towards the Sun.

( Sun reaches its highest position in the sky in a given hemisphere.)

Longest period of daylight.

Where is the Solstice vertically above?

head at Tropic of Cancer/Capricorn

Summer Solstice (N-Hemisphere)

June 21-22

Summer Solstice (S- hemisphere)

Dec 21-22

Tropic of Cancer/Capricorn

23.5 degrees latitude (same as the axial titlt angle)

highest/lowest latitudes

controlled by the axial tilt

Sun will never be exactly overhead

when more North or more South of those latitudes

Equinox

when the Sun is vertically above your head on the Equator.

Twice a year

Vernal equinox

equinox before summer.

Vernal equinox (N-hemisphere)

March 21-22

Vernal equinox (S-hemisphere)

Sept 22-23

Autumnal Equinox

equinox before winter.

Autumnal equinox (N- hemisphere)

Sept 22-23

Autumnal equinox (S- hemisphere)

March 21-22

Moon orbits

around the Earth

Moon orbital period

around 27 days

Moon rotates about its own axis

with a period of 27 days!

Moon's orbital period equals its rotational period

object takes the exact same amount of time to spin once on its own axis as it does to complete one full trip around the body it is orbiting.

Tidal locking

gravitational phenomenon that forces these two periods to match.

What does this mean for the moon?

Ultimately, same side of moon always faces us!!!

Looking down at the North pole

Earth orbits and rotates counterclockwise.

Moon orbits and rotates counterclockwise

Waxing

growing larger

Wanning

getting smaller

Crescent

Moon phase in which less than half of the Moon appears to be illuminated.

Gibbous

Moon phase in which more than half of the Moon appears to be illuminated.

Solar Eclipses

When the moon passes between the Sun and the

Earth in a way that the Moon blocks the light from the Sun.

Causes moon's shadow to fall on the Earth

Moon's orbit is tilted

5 degrees compared to the plane of Earth's orbit around the Sun

So the moon passes either above or below the plane

during a full moon or a new moon

Lunar eclipse

happens when the moon falls into the shadow of the Earth

Earth is blocking the Sun

Umbra

deep inner part of a shadow

Penumbra

partial shadow around the edges.

There are umbras and penumbras.

When a light source is extended

Moon's angular size

same as the Sun's angular size

Angular size

how we perceive the size of objects

Actual size

distance x angular size

What does angular size mean

they look the same size to us even though the Sun is larger (but it

is farther) and the Moon is the size of North America (but much closer)

Total/ full solar eclipse

standing in umbra

Partial eclipse

standing in penumbra

Tropical Year

365.24 days

A common year

365 days

Leap Years

every 4 years, the Earth's orbit is "behind" by one day. We allow a

leap day to let the Earth "catch up (366 days)

Celestial coordinates

locate where things are in the sky.

with respect to the (unchanging) night sky

with respect to our view from the ground (changing)

Equatorial Coordinate system (Right ascension-Declination system)

projection of the Earth's latitude and longitude system

onto the celestial sphere

Right ascension

similar to longitude (how much East or West)

Declination

similar to latitude (how much North or South)

Equatorial coordinates (RA-DEC)

An object in the night sky given by its celestial coordinates (RA, right ascension and DEC, declination) will. always have that value no matter what the time or location you are observing from

RA-Dec system

stays "attached" with the celestial sphere

Even as the Earth rotates

the coordinates of celestial objects like

stars, nebulae, etc do not change in

the RA-Dec system

Horizontal coordinate system (Altitude-

Azimuth system)

Local system. Based on where you are

particularly located. Based off your horizon.

Since ight sky rotates

an object's coordinates change over time!

Altitude

how high above the horizon something is (0-90 degrees)

Zenith

90 degrees altitude

Azimuth

how much East or West something is from the North

direction

Alt-Az system.

used for local astronomy, finding objects at

the moment with a telescope.

the Alt-Az coordinates of objects change over time

as the celestial sphere appears to rotate around you as time passes.

Light from the sun is made up

of all visible wavelengths

As light enters the atmosphere,

the blue wavelengths are scattered/reflected in all directions by the molecules in the atmosphere

The other wavelengths go through

unhindered and reach our eyes (making the Sun seem

mostly white, with a lack of a bit of blue...i.e.

more yellowish)

Scattered blue

seems to come from all directions thus making the sky look blue

When the Sun is low on the horizon the light

from the Sun passes through more atmosphere before reaching your eyes

Blue light is scattered

leaving only the orange and red parts of the

wavelengths remaining

The sun looks

reddish- orange during sunset as a result.