The Earth❖ Spherical body that is flattened near the poles due to centrifugal
force (rotation of the Earth)
‣ 40,074 KM across at the Equator
‣ 40,0007 KM across at the Poles
❖ Rotation: spinning of Earth around its axis (~24 hours to complete)
❖ Revolution: movement of Earth around the Sun (~365.25 days to complete)
Barycenter❖ The center of mass of two or more celestial
bodies that are orbiting one another
❖ This is the point around which the bodies orbit
❖ Point is based on the masses of the objects and distance between them
r1 = distance between body 1 & the barycenter m1 = mass of body 1 m2 = mass of body 2
a = distance between the centers of the two bodies
The Earth❖ Axis: imaginary line that runs through Earth from the North Pole to the South Pole, on which the
planet rotates around
❖ Earth’s axis is tilted an average of 23.5°
❖ Nutation: wobbling of the tilt of Earth’s axis, which can strengthen or weaken the effects of our seasons
‣ Tilt can increase to 24° or decrease to 22.5°
‣ Maximum cycle of nutation occurs over an 18 year period, due to the Moon’s effects
The Earth❖ Precession: change in the pointed direction of Earth’s axis
‣ Cycle is completed once every 26,000 years
‣ Does not affect intensity of our seasons, but only the time of year they occur
‣ Changes the North Pole stars
๏ Polaris currently (reaches max on March 24, 2100)
๏ Thuban was the N. Pole star around 3000 BC (one-fifth the brightness of Polaris)
๏ Alpha Cephei will be the next star, around 5000 AD
The Seasons❖ Equinox: 12 hours of daylight & 12 hours of darkness (Sun is overhead at the
Equator)
‣ Vernal equinox is currently March 21-22
‣ Autumnal equinox is currently September 21-22
‣ Will be reversed in about 12,000 years due to precession
The Seasons❖ Summer Solstice
‣ Occurs June 21-22
‣ Longest day of the year in the N. Hemisphere
‣ Sun is directly overhead the Tropic of Cancer (~23.5°N)
‣ 24 hours of sunlight for six months at the North Pole
❖ Winter Solstice
‣ Occurs December 21-22
‣ Shortest day of the year in the N. Hemisphere
‣ Sun is directly overhead the Tropic of Capricorn (~23.5°S)
‣ Zero hours of sunlight for six months at the North Pole
Will be reversed in about 12,000 years due to precession!
What about the Moon?❖ A satellite that orbits the Earth (our only natural satellite)
❖ Its gravity is 1/6th of Earth (too weak to hold an atmosphere for itself)
❖ Temperatures reach 134°C in sunlight and -170°C on the dark side of the Moon
❖ Its rate of rotation = its rate of revolution…also called “tidal lock”
Phases of the Moon❖ Full
❖ Waning Gibbous
❖ Last Quarter Half
❖ Waning Crescent
❖ New
❖ Waxing Crescent
❖ First Quarter Half
❖ Waxing Gibbous
Tidal Effects❖ Both the Moon & the Sun affect
Earth’s oceans
❖ Spring Tide: all three bodies (Earth, Moon, & Sun) are lined up, causing high tides to be higher and low tides to be lower
❖ Neap Tide: the bodies are at 90° angles, causing high tides to be lower and low tides to be higher
Eclipses
❖ Lunar eclipse occurs when the Moon travels through Earth’s shadow
❖ Solar eclipse occurs when the Earth travels through the Moon’s shadow
What is an ellipse?❖ A geometric shape with 2 focal points (a circle
has just 1 focal point)
❖ Has two axes (major & minor)
❖ Perihelion = when a planet is closest to the Sun
❖ Aphelion = when a planet is farthest from the Sun
Kepler’s Laws History❖ Tyco Brahe (1546-1601) was a Danish astronomer who found his
measurements showed that Mars did not orbit in a perfect circle
❖ Johannes Kepler (1571-1630) discovered that the orbit of Mars instead fit an elliptical shape
Kepler’s Laws
❖ 1st Law: orbital paths are ellipses and the Sun is at one focal point (nothing is at the 2nd point)
Kepler’s Laws❖ 2nd Law: the line joining the planets to the Sun sweeps out “equal
areas in equal times” as the planet travels around the ellipse
❖ This is due to a planet’s changing revolutionary speed during orbit (faster when close to the Sun & slower when farther from the Sun)