Cycles of the Sky

1
Cycles of the Sky

• Seeds chapter 3

2
Objective

• To understand the cyclic motions of the sun and
  moon

3
The Cycle of the Sun

• Earths rotation turning on an axis with
  respect to the Sun (a day)
• Earths revolution make one orbit of the Sun (a
  year)

4
The Annual Motion of the Sun

• The Earths orbit projected onto the sky makes
  the sun appear to follow a path called the
  ecliptic
• Sun moves about twice its own diameter per day
• 1 year 365.25 days

5
The Earths motion

• Vernal (March) equinox March 21
• Autumnal (September) equinox September 22
• Summer solstice
• Winter solstice

6
Orbital inclination

• Earths axis of rotation is fixed as we orbit the
  sun. The southern hemisphere is thus tilted away
  from the sun in June towards it each December.

7
The Seasons

• The ecliptic
• crosses the celestial equator at the equinoxes
• gets farthest south at our summer solstice
• gets farthest north at our winter solstice

8
The Suns daily path

• Solar diurnal motion
• is higher in the sky during summer
• lower in winter
• NB The sun moves across the northern sky in the
  southern hemisphere!

9
The spreading of sunlight

• A lower sun angle means the ground gets less
  energy per unit area cooler weather

10
The Motion of the Planets

• Mercury Venus are best seen at sunrise or
  sunset
• The outer planets can be seen in the night sky
• All planets appear near the ecliptic plane

11
The Cycles of the Moon

• Full Moon
• New Moon
• First Quarter
• Last Quarter

12
Moonphase

• The moons phases and sky position can be
  predicted using knowledge of its orbit.

13
Moonwatching

• The Moon also roughly follows the ecliptic and
  its daylight side faces the Sun.
• Find examples of artists getting the moon upside
  down!

14
Sidereal versus Synodic

• The Earths motion around the Sun makes the time
  from a full moon to the next (a synodic period of
  29.53d) longer than an orbit (a sidereal period
  of 27.322d)

15
Tides

• The Moon Suns gravity causes tides in our
  ocean, atmosphere and crust.
• Spring tides
• Neap tides

16
Tidal friction

• Earths rotation drags tidal bulges forward due
  to friction. This
• Lengthens day (2.3ms/century18h day 900Myr ago)
• Pulls moon forward, expanding its orbit (4cm/yr)

17
Lunar eclipses

• Umbra (total shadow )
• Penumbra (partial)

18
Eclipsed moon

• The Moon illuminated by all the sunsets of Earth

19
Lunar eclipse

• Occurs near where the moons orbit crosses
  ecliptic

20
Total or partial lunar eclipse

• Total Moon passes through Earths penumbra
  umbra
• Partial only through penumbra

21
Solar eclipse

• Suns surface obscured by the Moon
• Sun and Moon the same apparent size - a curious
  coincidence
• We see photosphere, chromosphere, corona
• Diamond ring effect

22
Solar eclipse

• Occurs near to where the ecliptic and the moons
  orbit cross
• We are in the moons penumbral or umbral shadow

23
Solar eclipse watching

• Partial solar eclipse can be watched safely using
  the projection method

24
Total solar eclipse

• The width and location of the Moons umbra
  defines the path of totality

25
Annular eclipse

• The moons umbra does not reach the Earths
  surface - moon is towards outer part of its
  slightly elliptical orbit

26
Eclipse seasons Saros cycle
27
Astronomical Influences on Earths Climate

• Milankovitch Ice ages are caused by changes in
  Earths
• orbit
• precession
• inclination