Chapter 1: History of the Sky


  • All ancient cultures observed the cycles of the sky.
  • Ex. Stonehenge.

Astronomical Cycles

  • Many astronomical events repeat in regular cycles (ex. Phases of the moon).
  • The repetition of cycles help us understand them.
  • Some cycles have been useful - timing of seasons, migration of animals - these were astronomy’s practical contributions.

Celestial Sphere

  • The stars in space are in different directions and at different distances - space is 3-dimensional.
  • However, stars are so far from Earth they all appear to be at the same distance.
  • Therefore, we imagine the stars attached to the surface of a gigantic sphere surrounding Earth - “celestial sphere” this is a convenient “model” of the sky.
  • NOTE: we are at the centre of the celestial sphere, but we draw it as though we are outside it - which is impossible.
  • Wherever we are on Earth, Earth blocks half the sky from our view.
  • We call the boundary between Earth and the visible sky the “horizon” (ignoring trees, buildings, mountains.


  • To organize the sky, we imagine images that sort of match the pattern of stars -== “constellations” - see the maps in the text.
  • Because stars are so far away, the constellations don’t change for many thousands of years, even though the stars are moving at high speeds.
  • From a different solar system the constellations would look different.
  • The stars in a constellation are usually not physically close to each other, they just appear close together to us.
  • In this course the constellations are only used to provide a background reference to track the motions of the members of the solar system.

Daily Motions in the SKY

  • Every day we see:
  • The Sun rises from the eastern horizon.
  • The Sun moves westward across our sky.
  • The Sun sets below the western horizon.
  • The Moon, planets, and stars also rise, move across the sky and set every day.
  • These are all caused by Earth rotating (or spinning) once every day toward the East.

Sun’s Yearly (or Annual) Motion

  • In addition to its daily rising and setting, the Sun also appears to move around the celestial sphere every year.
  • Problem: the Sun’s brightness blocks out the background stars - how do we know?
  • Look at the western horizon after sunset.
  • Look overhead at midnight.
  • Sun appears to move toward the east, cause by Earth’s orbit around the Sun.
  • The Sun’s path on the celestial sphere, called the ecliptic.


  • Common misunderstanding - people think seasons are caused by the earth’s distance from the Sun - wrong! How do we know?
  • July is summer in Ontario but winter in Australia.
  • In July the Sun appears smaller - we are farther from the Sun.
  • In January the Sun appears larger - we are closer to the Sun.
  • Seasons are caused by the tilt of Earth’s spin axis.
  • NOT perpendicular to Earth’s orbit direction.
  • Tilt is 23.5 degrees to Earth’s orbit direction.
  • Summer happens when our hemisphere is tilted toward the Sun.
  • Sunlight is more concentrated on the ground.
  • The Sun is up for more than 12 hours.
  • Special seasonal dates:
  • Summer solstice is about June 21 plus or minus a few days.
  • “Solstice” - Sun stops moving higher in the sky because Earth has its maximum tilt toward the Sun.
  • Autumnal equinox is about September 22 plus or minus a few days.
  • “Equinox” - hours of daylight - hours of night.
  • Winter solstice is about December 21 plus or minus a few days.
  • Sun stops moving lower in the sky.
  • Spring equinox - March 20 plus or minus a few days.

The Moon

  • Only solar system object that orbits Earth
  • How do we know?
  • Moon’s orbit period can be measured using the stars as a reference.
  • About 27.3 days.

The Moon’s Phases

  • The moon does not glow and emit light, it only scatters sunlight.
  • The moon has a bright sunlit dayside and a dark nightside.
  • New phase - dayside of Moon is away from Earth \rightarrow we don’t see anything.
  • Waxing crescent phases.
  • “Waxing” == “increasing” amounts of the Moon’s sunlit side are visible from Earth.
  • First quarter phase.
  • We see half of the Moon’s dayside and half of its nightside. “Quarter” =14= \frac{1}{4} of the cycle of phases.
  • Waxing gibbous phases - we see more than half of the dayside of the Moon and it increases each night.
  • Full Moon == we see the full dayside.
  • Waning gibbous phases: “waning” -== decreasing amount of the Moon’s dayside.
  • Third quarter phase =34= \frac{3}{4} of the cycle.
  • Waning crescent phases.
  • The Moon’s cycle of phases takes about 29.5 days, which is close to the length of our month.
  • But using the star constellations we found the Moon orbits Earth in 27.3 days.
  • Why are these periods different?
  • The 27.3 day period found using the star constellations is the true orbit needed for the Moon to orbit 360 degrees around Earth.
  • The 29.5 day cycle of phases combines the Moon’s orbit around Earth with Earth’s orbit around the Sun
  • Each particular phase requires the Moon, Earth, and Sun to have the same particular relative positions, but Earth is orbiting.

Phases and Times of Day

  • Each phase of the Moon depends on its position relative to the Sun as seen from Earth.
  • Therefore, a particular phase can only be observed at particular times of the day.
  • The Moon at a particular phase rises at a certain time, passes overhead at a certain time, and sets at a certain time.

What Causes Eclipses?

  • When the Moon is at its new phase it can block the SUn’s light \rightarrow solar eclipse.
  • At full phase the Moon can pass through Earth’s shadow \rightarrow lunar eclipse.
  • NOTE: the lunar eclipse is NOT black because:
  • Earth’s atmosphere reddens the sunlight.
  • Bends the light into the shadow.

How Long Do Eclipses Last?

  • Total lunar eclipses last about an hour as the Moon orbits through Earth’s shadow.
  • Total lunar eclipses are visible to everyone on Earth night side.
  • Total solar eclipses last no longer than about 7 minutes as the Moon orbits in front of the Sun.
  • Total solar eclipses are only visible in the Moon’s small shadow as it sweeps across Earth.

Why are Eclipses Rare?

  • Every cycle of 29.5 days the Moon has a new phase and a full phase.
  • But, solar and lunar eclipses do NOT occur every month - Why?
  • Moon’s orbit is tilted compared to Earth’s orbit which prevents exact alignment even though the phase is correct.
  • When the Moon is crossing Earth’s orbit at the correct phase \rightarrow an eclipse can occur = eclipse seasons - 21 January 2019.

Other Kinds of Eclipses

  • If the alignment is close but not exact, the eclipse is “partial”
  • Only part of the Sun is blocked.
  • The Moon passes through only part of Earth’s shadow.
  • If the Moon is slightly farther from Earth or if Earth is slightly closer to the Sun, the Moon is too small to cover the Sun.
  • An “annular eclipse”.

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