# Chapter 1: History of the Sky

## History:

• 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.

### Constellations

• 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.
• 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.

### Seasons

• 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.

#### 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” ﻿$= \frac{1}{4}$﻿ 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 ﻿$= \frac{3}{4}$﻿ 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”.