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Unit 1Movement of the Earth, Seasons, Coordinates and Observing the night sky
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Stars seem to be on the inner surface of a sphere surrounding the Earth.
Celestial Sphere
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Stars that appear close in the sky may not actually be close in space.
Celestial Sphere
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Celestial Poles are points on the celestial sphere north and south of the Earth’s north and south poles.
Celestial Sphere
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Imaginary circle on the celestial sphere directly above the Earth’s equator.
Celestial Equator
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Celestial Coordinates• Horizon – Edge of our
local sky– We can only see half of the
celestial sphere at one time.
• Zenith – The point straight overhead on the celestial sphere.– What you see if you lie on
your back and look straight up
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• Altitude – How many degrees above the horizon• Azimuth – How many degrees along the horizon from the north
Celestial Coordinates
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• Declination (Dec)• Degrees north or
south of celestial equator
• Right ascension (Ra)• Measured in hours,
minutes, and seconds eastward from position of the Sun at vernal equinox
Celestial Coordinates
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Celestial CoordinatesCompare and contrast Ra, Dec, Latitude and Longitude.
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Ecliptic• The apparent path
of the sun on the celestial sphere over the course of a year.
• Tilted at an angle of 23.5° with respect to the celestial equator
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The 12 constellations the Sun moves through during the year are called the zodiac
Location of all planets when they are visible in the sky.
Ecliptic
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Diurnal MotionApparent rising andsetting of stars
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• Revolution – One body moves around another
• Rotation– Movement of an object around its axis
Earth’s Orbital Motion
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• Solar Day – Our observed daily cycle (Noon to Noon)
• Sidereal Day – Time it takes for a star to be in the exact same position in the sky. (23.93 hours)
Earth’s Orbital Motion
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Summer Solstice is June 21 – Sun at it’s northernmost point (above celestial equator) Winter Solstice is December 21 – Sun at it’s southernmost pointThis is caused by the tilt of the earth on it’s axis.
Responsible for seasons (not distance from Sun)
Points where path of the sun crosses celestial equator are vernal (March 21) and autumnal (Sept 21) equinoxes. Tropical Year = 365 solar days
Interval of time from one equinox to the next
Earth’s Orbital Motion
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Precession: Rotation of Earth’s axis itselfmakes one complete circle in about 26,000 years
Earth’s Orbital Motion
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Earth’s Orbital Motion
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A sidereal year is the time it takes for Earth to orbit once around the sun relative to fixed stars.Tropical year follows seasons (365 days)
Sidereal year follows constellations (365.256363004 days)In 13,000 years July and August will still be summer, but Orion will be a summer constellation.
Why we have leap year!
Earth’s Orbital Motion
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Moon Phases
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Phases are due to different amounts of sunlit portion being visible from Earth.
Moon Phases
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Synodic Month – Time it takes the moon to go through a whole cycle of phases (29.5 days).
The Motion of the MoonSidereal Month - Time to make full 360°revolution around Earth (about 2 days shorter than synodic month).
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Tides are due to gravitational force on Earth from the Moon The force on near side of Earth is greater than the force on far side. Water can flow freely in response.
The Tides
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The Sun has less effect, but it does modify the lunar tides.Spring tides- more tidal differentiationNeap tides – less tidal differentiation
The Tides
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Tides tend to exert a “drag” force on Earth, slowing its rotation.This will continue until Earth rotates synchronously with the Moon, so that the same side of Earth always points toward the Moon.
The Tides
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This has already happened with the Moon, whose near side is always toward Earth.
The Tides
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Occurs when Earth is between the Moon and Sun
• Partial when only part of the Moon is in shadow (Penumbra)
• Total when all is in shadow (Umbra)
Lunar Eclipse
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Lunar Eclipse
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Occurs when the Moon is between Earth and Sun
Solar Eclipse
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Solar Eclipse
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Annular Solar Eclipse
Moon is too far from Earth for total.
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Eclipses don’t occur every month because Earth’s and the Moon’s orbits are not in the same plane.
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Solar Eclipse Tracks, 2010 - 2030
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Measure baseline and angles, and you can calculate distance.
Tan = opp / adj
Tan B = Distance to Object / baseline
Distance to Object = (Tan B)(Baseline)
Triangulation
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Similar to triangulation, but looking at apparent motion of object against distant background from two vantage points
Parallax
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The greater the parallax, the closer the star
Parallax
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Angular Measure
The angular diameter is proportional to the actual diameter divided by its distance. If any two of these quantities are known, the third can be determined.
The basic unit of angular measure is the degree (°).
Astronomers use angular measure to describe the apparent size ofa celestial object
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Angular Measure
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Stars can be imagined to be on inside of celestial sphere; useful for describing location.
Plane of Earth’s orbit around Sun is ecliptic; at 23.5° to celestial equator.
Angle of Earth’s axis causes seasons. Moon shines by reflected light, has phases. Solar day ≠ sidereal day, due to Earth’s rotation
around Sun. Synodic month ≠ sidereal month, also due to
Earth’s rotation around Sun
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Tropical year ≠ sidereal year, due to precession of Earth’s axis
Distances can be measured through triangulation and parallax.
Eclipses of Sun and Moon occur due to alignment; only occur occasionally as orbits are not in same plane.
Apparent size can be determined using angular measurement
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