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 WORDS RotateRevolveAxis SolsticeEquinoxPhase PlanetLight-year Astronomical 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 23.5 o (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

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

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 10 X ) Express very large or very small numbers “Power of 10” equals the number of places the decimal was moved (+) large, (-) small is 3.2 x is 5.5 x x 10 5 is x is

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 = 1.5x10 8 km Common Distances: Sun - Pluto: ~ 40 AU Sun - Saturn: ~ 10 AU Sun - Jupiter: ~ 5 AU Sun - Mars: ~ 1.5 AU So Pluto is 40x farther away from the Sun than Earth

 Nearest star - 4.1x10 13 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. 9.46x10 12 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. Centari4.3 Sirius 8.8 Betelguese 700 Rigel 900 Most 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 WORDS RotateRevolveAxis SolsticeEquinoxPhase PlanetLight-year Astronomical Units