Published 2 years ago
- All ancient cultures observed the cycles of the sky.
- Ex. Stonehenge.
- 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.
- 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.
- 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 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” 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 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 solar eclipse.
- At full phase the Moon can pass through Earth’s shadow 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 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”.