1. Unit 1 Movement of the Earth, Seasons, Coordinates and Observing the night sky

2. Celestial Sphere
Stars seem to be on the inner surface of a sphere surrounding the Earth.

3. Celestial Sphere
Stars that appear close in the sky may not actually be close in space.

4. Celestial Sphere
Celestial Poles are points on the celestial sphere north and south of the Earth’s north and south poles.

5. Celestial Equator
Imaginary circle on the celestial sphere directly above the Earth’s equator.

6. 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

7. Celestial Coordinates
Altitude – How many degrees above the horizon
Azimuth – How many degrees along the horizon from the north

8. Celestial Coordinates
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

9. Celestial Coordinates
Compare and contrast Ra, Dec, Latitude and Longitude.

10. 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

11. Ecliptic
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.

12. Diurnal Motion
Apparent rising and
setting of stars

13. Earth’s Orbital Motion
Revolution – One body moves around another
Rotation– 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 point
This 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

16. Earth’s Orbital Motion
Precession: Rotation of Earth’s axis itself
makes one complete circle in about 26,000 years

17. Earth’s Orbital Motion

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!

19. Moon Phases

20. Moon Phases
Phases are due to different amounts of sunlit portion being visible from Earth.

21. The Motion of the Moon
Sidereal 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 differentiation
Neap tides – less tidal differentiation

24. The Tides
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.

25. The Tides
This 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)

27. Lunar Eclipse

28. Solar Eclipse
Occurs when the Moon is between Earth and Sun

29. Solar Eclipse

30. Annular Solar Eclipse
Moon is too far from Earth for total.

31. Eclipses don’t occur every month because Earth’s and the Moon’s orbits are not in the same plane.

32. Solar Eclipse Tracks,

33. Triangulation Tan = opp / adj
Measure baseline and angles, and you can calculate distance.
Tan = opp / adj
Tan B = Distance to Object / baseline
Distance to Object = (Tan B)(Baseline)

34. Parallax
Similar to triangulation, but looking at apparent motion of object against distant background from two vantage points

35. Parallax
The 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 of
a celestial object
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.

37. Angular Measure

38. Review
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

39. Review
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