Seasons and Change

S1-4-05a Explain the apparent motion of the Sun, stars, planets, and the Moon as seen from Earth.

S1-4-06a Differentiate between units of measure used for astronomical distances.

KEY WORDSRotate Revolve AxisSolstice Equinox Phase

Planet Light-yearAstronomical Units

The Earth rotates on its axis once a day.At any time - half of Earth is in “day” and half in “night.”

Earth rotates towards the East

The Earth revolves around the Sun once a year. At any time - half of Earth is in “winter” and half in “summer.”

summer

winter

Time-lapse photography to show the stars spinning as we rotate on our axis

Our axis is tilted at 23.5o (degrees)

This tilt and the curvature of the Earth is responsible for the variations in the Sun’s energy

– and creates our seasons.

Summer“direct” energy

Winter“indirect” energy

Summer: not only more “direct” but also more hours of the Sun’s energy

Summer Solstice (June 21)

Longest Day

Winter Solstice (Dec 21)

Shortest Day

Fall Equinox (Sept 22)

12h and 12h

Spring Equinox (Mar 21)

12h and 12h

WHAT DO YOU NOTICE?

• Moon completes 1 rotation on its axis in the same time as it completes 1 revolution around the Earth (synchronous)

• We always see the same side facing the Earth

• Orbit is elliptical• Tilted 5°• Mean Distance:

384,400 km

Moon Rotation

That’s why there is a “dark side” of the Moon

Phases of the Moon• Shines only by reflected sunlight• Phase depends on the fraction of the sunlit

hemisphere visible to us.

Half facing the Sun is fully lit

Opposite hemisphere is always dark

http://www.moonconnection.com/

Our solar system:Sun (a star) and all the objects in orbit around it.

Scientists estimate our solar system is 4.6 billions years old

Remember, we can see moons and planets because light from the sun reflects off them

My Very Educated Master Just Showed Us Nine Planets

Planet: a large collection of matter that is generally spherical and revolves around a star.

• Planets / moons are nonluminous (don’t emit light)

Scientific Notation (x 10X)• Express very large or very small numbers• “Power of 10” equals the number of places

the decimal was moved (+) large, (-) small

32 000 000.0 is 3.2 x 107

0.0000000055 is 5.5 x 10-9

2.6 x 105 is 260 000.02.6 x 10-5 is 0.000026

Distance and the Universe• Common to use Astronomical Unit (A.U.)

1 A.U. = distance between the Earth and the Sun

Sun to the Earth: 1 AU = 1.5x108 km

Common Distances:Sun - Pluto: ~ 40 AU Sun - Saturn: ~ 10 AUSun - Jupiter: ~ 5 AU Sun - Mars: ~ 1.5 AU

So Pluto is 40x farther away from the Sun than Earth

Nearest star - 4.1x1013 km away from Earth!

This star is Proxima Centari

Light-year (LY):•The distance a beam of light travels in one year

Light moves outward fast – about 300,000 km/sec.•9.46x1012 km /year = 1 light-year•P. Centari is 4.3 light-years away

That’s 41,000,000,000,000 km!

Distances can be deceiving: Bright stars look close, but may be very far away

Star

Approx. Distance (LY)P. Centari

4.3Sirius

8.8Betelguese

700Rigel

900Most distant known galaxy15,000,000,000

Star light takes years to get to Earth – this delay means we are looking at old “images” – it’s like

looking into the past…

CAN YOU ANSWER THESE QUESTIONS?

S1-4-05a:How does the position and motion of Earth produce day/night, the phases of the moon, and the seasons?

S1-4-06a:How do astronomers measure the great distances in the universe?

KEY WORDSRotate Revolve AxisSolstice Equinox Phase

Planet Light-yearAstronomical Units