Presentation on theme: "Celestial Sphere Stars seem to be on the inner surface of a sphere surrounding the Earth."— Presentation transcript:
1 Unit 1 Movement of the Earth, Seasons, Coordinates and Observing the night sky
2 Celestial SphereStars seem to be on the inner surface of a sphere surrounding the Earth.
3 Celestial SphereStars that appear close in the sky may not actually be close in space.
4 Celestial SphereCelestial Poles are points on the celestial sphere north and south of the Earth’s north and south poles.
5 Celestial EquatorImaginary circle on the celestial sphere directly above the Earth’s equator.
6 Celestial Coordinates Horizon – Edge of our local skyWe 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
7 Celestial Coordinates Altitude – How many degrees above the horizonAzimuth – How many degrees along the horizon from the north
8 Celestial Coordinates Declination (Dec)Degrees north orsouth of celestialequatorRight ascension (Ra)Measured in hours,minutes, and secondseastward fromposition of the Sun atvernal equinox
9 Celestial Coordinates Compare and contrast Ra, Dec, Latitude and Longitude.
10 EclipticThe 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
11 EclipticThe 12 constellations the Sun moves through during the year are called the zodiacLocation of all planets when they are visible in the sky.
12 Diurnal MotionApparent rising andsetting of stars
13 Earth’s Orbital Motion Revolution – One body moves around anotherRotation– Movement of an object around its axis
14 Earth’s Orbital Motion 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)
15 Earth’s Orbital Motion 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 daysInterval of time from one equinox to the next
16 Earth’s Orbital Motion Precession: Rotation of Earth’s axis itselfmakes one complete circle in about 26,000 years
18 Earth’s Orbital Motion 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 ( days)In 13,000 years July and August will still be summer, but Orion will be a summer constellation.Why we have leap year!
20 Moon PhasesPhases are due to different amounts of sunlit portion being visible from Earth.
21 The Motion of the MoonSidereal Month - Time to make full 360°revolution around Earth (about 2 days shorter than synodic month).Synodic Month – Time it takes the moon to go through a whole cycle of phases (29.5 days).
22 The Tides 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.
23 The Tides The Sun has less effect, but it does modify the lunar tides. Spring tides- more tidal differentiationNeap tides – less tidal differentiation
24 The TidesTides 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.
25 The TidesThis has already happened with the Moon, whose near side is always toward Earth.
26 Lunar Eclipse 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)
33 Triangulation Tan = opp / adj Measure baseline and angles, and you can calculate distance.Tan = opp / adjTan B = Distance to Object / baselineDistance to Object = (Tan B)(Baseline)
34 ParallaxSimilar to triangulation, but looking at apparent motion of object against distant background from two vantage points
35 ParallaxThe greater the parallax, the closer the star
36 Angular Measure The basic unit of angular measure is the degree (°). Astronomers use angular measure to describe the apparent size ofa celestial objectThe 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.
38 ReviewStars 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
39 ReviewTropical year ≠ sidereal year, due to precession of Earth’s axisDistances 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