AstronomyAstronomy

Celestial Celestial ObservationsObservations

The Celestial SphereThe Celestial Sphere

All celestial objects, (sun, stars, moon, planets), from Earth appear to be on the surface of an imaginary sphere surrounding Earth.

This imaginary sphere is called the

Celestial Sphere.Celestial Sphere.

Celestial Observation Celestial Observation TermsTerms

HorizonHorizon = where celestial sphere meets Earth.

ZenithZenith = point on celestial sphere directly above observers position.

AltitudeAltitude = objects distance, in degrees, above horizon.

AzimuthAzimuth = objects distance, in degrees, measured clockwise from due north position (due north is 00 azimuth).

Star PathsStar Paths

All celestial objects appear to move from east to west across the sky in an arc.

Why?Why?Circumpolar starsCircumpolar stars move in

counterclockwise circles between the northern horizon and Polaris and never set.

RotationRotation

The apparent daily motion of celestial objects is due to Earth’s rotationrotation.

RotationRotation is the turning of an object on its axis.

Celestial objects are not actually moving from east to west across the sky, rather the earth is rotatingrotating under them.

Rotations

RevolutionRevolution

RevolutionRevolution is the movement of one celestial object around another.

The path along which an object travels during a revolution is called an orbitorbit.

Earth’s RevolutionEarth’s Revolution

Orbits

What are What are ConstellationsConstellations??

ConstellationsConstellations are stars that appear to be grouped in patterns that form the outlines of “things” (people, animals, objects).

The night sky is divided into 88 constellationsconstellations.

Circumpolar constellationsCircumpolar constellations are those constellations that are always visible as they never set below the horizon.never set below the horizon.

Star ShiftsStar Shifts

Constellations shift, rise and set, (similar to the rise and set of the Sun) during a night due to the rotation of the Earth.

Constellations in the sky slowly change from one night to the next as a result of the revolution of Earth around the sun.

Constellations rise approximately 4 minutes later each night.

Planetary Motions

• On a daily basis planets and stars appear to move from east to west.

• Over long periods of time the planets appear to move eastward relative to the stars behind them.

Celestial ModelsCelestial ModelsGeocentric

Theory: this theory, put forward by the ancient Egyptian astronomer Ptolemy, states that the Earth is the center of the universe.

Heliocentric Theory: this theory, put forward by the Polish scientist Copernicus, states that the Sun is the center of the universe.

Ptolemy and Geocentric Theory

Copernicus and Heliocentric Theory

Orbital ForcesOrbital Forces

Johann Kepler developed the “Laws of Planetary Motion” which states that planets orbit in ellipsesellipses.

Unlike a circle the center of an ellipse consists of two fixed points called focifoci.

Kepler’s First Law:The orbits of the planets around the Sun

are ellipsesellipses, with the Sun at one of the focifoci.

Kepler’s LawsKepler’s LawsKepler’s second law states that:An imaginary line joining a planet to

the Sun will sweep over equal areas in equal periods of time.

PerihelionPerihelion = when the planet is closestclosest to the Sun (moving the fastestfastest).

AphelionAphelion = when the planet is farthestfarthest from the Sun (moving the slowestslowest).

Evidence of Earth’s Evidence of Earth’s RotationRotation

• Foucault PendulumFoucault Pendulum: the greater the latitude the greater the hourly change in direction.

• Coriolis EffectCoriolis Effect:: is the tendency of matter moving across the Earth’s surface to be deflected from a straight-line path.

• Other evidence includes the day night cycle, star paths and satellite photos.

Apparent Motions of the Apparent Motions of the SunSun

• Due to the rotation of the Earth, Due to the rotation of the Earth, the Sun appears to move in an the Sun appears to move in an arc across the sky from east to arc across the sky from east to west.west.

• The tilt of the Earth’s axis, 23.5The tilt of the Earth’s axis, 23.500, , causes the location of the direct causes the location of the direct rays of the Sun to change in a rays of the Sun to change in a cyclic pattern.cyclic pattern.

Change of SeasonsChange of Seasons

• Three factors cause the seasonal changes:– Earth’s revolution around the sun– Tilt of the Earth’s axis– Parallelism of the axis

Time and Earth MotionsTime and Earth Motions• Sidereal daySidereal day is the time period for

Earth to make one complete rotation on it’s axis (23 hrs, 56 min, 4 sec.).

• Apparent solar dayApparent solar day is about 4 min longer (based on solar noon to solar noon) to account for the additional rotation needed as a result of Earth’s revolution around the sun.

• Since our orbital speed varies, therefore the apparent solar day varies, the mean solar daymean solar day (24 hrs) is used for daily timekeeping.

Moon MotionsMoon Motions

• Moon orbits earth once every 27 1/3 days.• The barycenterbarycenter is the gravitational center

the Earth and Moon revolve around (1700km beneath surface of Earth).

• Sidereal month (27 1/3 days) is time for the Moon to revolve around the Earth.

• Synodic month (29 ½ days) is the time for the Moon to complete a cycle of phases.

Phases of the MoonPhases of the Moon• The illuminated portion of the moon

that faces Earth varies in a cyclic pattern called phasesphases.

• New moonNew moon = when the entire illuminated portion is facing away from Earth.

• Waxing periodWaxing period = when right hand portion seems to grow more lighted.

• Full moonFull moon = when the entire illuminated portion is facing toward Earth.

• Waning periodWaning period = when the left hand portion shrinks in illumination.

EclipsesEclipses• Solar eclipseSolar eclipse occurs when the Moon

passes between the Earth and the Sun.– Umbra is the part of the Moon’s shadow in

total darkness– Penumbra is the part of the Moon’s

shadow in partial darkness.– Annular eclipse occurs when the Moon is

at apogee and the outer edge of the Sun is visible.

• Lunar eclipseLunar eclipse occurs when the moon passes through the Earth’s shadow.

Solar and Lunar Eclipse