Astronomy Notes: Seasons, Moon Phases, Eclipses, and Planets

Canvas Site and Course Logistics

App Canvas site is separate from the core Canvas site for the course; make sure you can access your lab Canvas site and all lab materials.
Submissions for lab work must go through the lab Canvas site, not the module in the My Course Canvas site.
This week’s plan: finish Chapter 2, and start Chapters 3 and possibly 4 on Thursday.
Reading quiz opened today (the material in the reading assignment); due next Monday right before midnight. Last week was a practice quiz; this week’s quiz will count toward your grade.
First homework was due yesterday; second homework (Chapters 2 and 3) due one week from yesterday.
Quick recap: Last class finished Chapter 1 and began patterns of the night sky; Chapter 2 covers the reasons for the seasons; this session covers the Moon and Moon phases and the ancient mystery of planets. Chapter 3 may be touched; Chapter 4 is aspirational.
Warm-up activity: bring your ABC cards; discuss Polaris and why it is special; many students initially guessed differently.

Polaris and the North Star

Polaris is special because it is near the axis about which the sky appears to rotate (Earth’s rotation axis).
The Earth rotates on an axis; the axis currently points toward Polaris, so Polaris appears to be in the north direction from the Northern Hemisphere.
The northward direction (azimuth) from any location in the Northern Hemisphere toward the north is toward Polaris.
The position of Polaris is not fixed historically; precession will change the pole’s pointing over long timescales.

Why Do Seasons Occur? The Subtler Point

The instinct that “the seasons are due to the tilt of the axis” is broadly correct but needs nuance.
The tilt of the Earth’s axis remains constant in direction relative to the distant stars, but as the Earth orbits the Sun we see different constellations behind us at night.
Correct answer (in class terms): the tilt of Earth’s axis causes seasons; which stars you see at night changes with the orbital position around the Sun.
For example: in August you might see Aquarius in the night sky; in February you’ll see Leo when you look away from the Sun.
Key point: A is not the right answer to the subtler question about seasons changing with background stars; the tilt stays the same, but the background stars change as Earth orbits the Sun.
The real mechanism: the axis tilt determines how sunlight is distributed and how long the Sun stays high in the sky; as Earth orbits the Sun, the Sun’s path in the sky changes, changing the seasons.
Important clarifications:
- The tilt is the essential driver of seasons, not the Earth–Sun distance (which is actually smaller in winter than in summer when measured relative to the Sun).
- The tilt produces more direct sunlight and longer days in summer, less direct sunlight and shorter days in winter.
- Summer solstice: Sun follows its highest path in the sky; longest day of the year (Northern Hemisphere).
- Winter solstice: Sun follows its lowest path; shortest day of the year (Northern Hemisphere).
The Earth–Sun distance fluctuates seasonally, but it is not the primary driver of seasons.
The discussion also touches on the Sun’s height and duration in the sky contributing to heat and warmth.

Solstices and Equinoxes; Daily Light Cycles

Summer Solstice: Sun at its highest path; longest day; around June 21.
Winter Solstice: Sun at its lowest path; shortest day; around December 21.
Equinoxes (equal day and night):
- Autumnal (Fall) Equinox: around September 22; day and night are about equal.
- Spring (Vernal) Equinox: around March 20; day and night are about equal.
At the solstices, the Sun rises and sets at extreme positions (north of east and north of west at summer; south of east and south of west at winter).
At the equinoxes, the Sun rises due east and sets due west, giving nearly equal daylight and night.
Polar phenomena:
- At the North Pole, the Sun can stay above the horizon during the summer solstice (midnight sun).
- At the South Pole (Antarctica), there can be continuous daylight for a long stretch during their summer; winters bring extended darkness.
- Practical note: in polar regions there can be extreme daylight conditions; for example, staying for months at a time to operate telescopes (e.g., ~25 Hardy Sols in the South Pole nets).
Cultural and historical notes: Polaris’s role as a navigational reference has shifted over millennia due to precession.

Precession and The Moving North Star

Polaris is not always the North Star; it is currently near the pole, but this will change as the axis precesses.
Precession: the slow wobble of Earth's rotation axis, like a spinning top, causing the axis to trace a circle over a ~26,000-year cycle.
This implies that the pole’s pointing direction changes over thousands of years; the “North Star” changes over time.
Example: ~4,000 years ago (Egyptian pyramids), Thuban was closer to the pole and served as the North Star.
By around 2100, Polaris will be closest to the rotational axis; after that, it will slowly move away from the exact pole.
Implications: navigation using a fixed North Star has changed through history; our technology now allows navigation without a bright fixed North Star.
Analogy used in lecture: compare the Earth’s axis to a top; precession is like the top’s axis slowly moving.

The Moon: Phases, Orbits, and Observing Geometry

Why the Moon has phases:
- The Moon orbits the Earth, and sunlight comes from the Sun. The angle between Sun–Moon–Earth determines how much of the Moon’s lit side we see.
- The Sun’s rays can be treated as nearly parallel for Earth–Moon distances, so illumination is essentially the same direction for both bodies at any given moment.
The Earth–Moon distance (as given in lecture): approximately $d \approx 3.8\times 10^{4} \,\text{km}$ (reported as 38,000 km in the transcript).
Everyday motion: the Moon rises in the east and sets in the west; each day it appears a bit farther east in the sky due to its orbit around the Earth.
The Moon’s cycle and the Sun’s cycle interact to create the lunar phases:
- New Moon: Moon is between the Sun and Earth; it is not visible.
- Waxing Crescent: a small crescent becomes visible after New Moon.
- First Quarter: the Moon is at ~90° from the Sun (Sun–Earth–Moon form a right angle); half of the Moon’s near side is illuminated.
- Waxing Gibbous: more than half visible.
- Full Moon: Sun–Moon–Earth are aligned in a straight line with the Earth in between; the entire near side is illuminated.
- Waning phases: after Full Moon, the visible illuminated portion decreases (waning gibbous → last/third quarter → waning crescent → New Moon).
Important clarifications about illumination:
- The Sun always illuminates half of the Moon; what changes is the fraction of the Moon we see illuminated from Earth.
- There is no permanently dark side of the Moon; all sides are illuminated at some time; we just don’t see the far side from Earth when it’s New Moon.
The Moon’s phase cycle length:
- Sidereal month (Moon’s orbit relative to the stars): about $27.3\text{ days}$ .
- Synodic month (Moon relative to the Sun as observed from Earth; cycle of phases): about $29.5\text{ days}$ .
Why is the synodic month longer than the sidereal month?
- Because the Earth-Moon system orbits the Sun; the Moon must travel a little further to reach the same phase alignment relative to the Sun.
The etymology of “month”: named after the Moon; roughly a monthly cycle of Moon phases.
The calendar issue (brief): calendars once used roughly 30-day months; February is shorter; Gregorian reform adjusted the calendar centuries ago; the Moon’s 29.5-day cycle isn’t perfectly aligned with the calendar.
Quick demonstration concept from lecture: a sun–Moon–Earth alignment diagram illustrates how the same Sun direction can illuminate different fractions of the Moon through the cycle.
Practical mnemonic recaps:
- In the Northern Hemisphere, from New Moon to First Quarter, the crescent appears on the right; from Full Moon to Last Quarter, the crescent appears on the left.
- The waxing phases are visible in the afternoon and evening; the waning phases are visible late at night and in the morning.
The orbit of the Moon and the Earth’s orbit around the Sun create a difference between sidereal and synodic periods; the term “month” historically reflects the Moon’s cycle.
Tidally locked detail (tidal locking): the Moon’s rotation period matches its orbital period due to tidal interactions; this is called tidal locking, resulting in the same lunar face always facing Earth.
Note: there is a distinction between the Moon’s near side (facing Earth) and far side (facing away); the “dark side” terminology is a misnomer; the far side is illuminated when it faces the Sun, just not toward Earth.

Eclipses: Shadows, Nodes, and Cycles

Basic eclipse concept: eclipses occur when shadows are cast by either the Sun (Sun–Moon–Earth alignment) or the Earth (Earth–Moon alignment) during specific alignments.
Types of shadows:
- Umbra: full shadow where the Sun’s light is completely blocked.
- Penumbra: partial shadow where only part of the Sun’s light is blocked.
Lunar eclipses:
- Occur only at Full Moon when Earth is between Sun and Moon and can cast its shadow on the Moon.
- Subtypes: penumbral, partial, total (depending on how much of the Moon passes through the Earth’s shadow).
Solar eclipses:
- Occur only at New Moon when the Moon is between the Sun and Earth and casts its shadow on Earth.
- Subtypes: partial, total, and annular. An annular eclipse occurs when the Moon is farther from Earth, so it does not completely cover the Sun, leaving a bright ring (an annulus).
Key spatial relationships:
- The Moon’s orbit is tilted relative to the ecliptic plane by about $5^{\u00b0}$ (five degrees) with respect to the plane in which the Earth orbits the Sun (the ecliptic).
- This tilt means eclipses do not occur every New or Full Moon; the Moon must be near the nodes where its orbit crosses the ecliptic to line up with the Sun and Earth.
Eclipse seasons:
- Occur roughly twice per year when the line of nodes aligns with the Sun-Earth line.
- Each eclipse season contains a lunar eclipse (at Full Moon) and a solar eclipse (at New Moon).
The Saros cycle (predicting eclipses):
- A long-term cycle of about $18\text{ years}$ (the Saros period) through which eclipses repeat with similar geometry.
- The Greeks historically predicted eclipses using this cycle, though predicting the exact geographic location of the eclipse path remains complex.
Observational notes and examples from recent history:
- In North America, several eclipses have been visible in recent years; the next major US-crossing solar eclipse is projected for $2045$ , and a partial lunar eclipse visible from Minneapolis in $2026$ .
Why eclipses don’t happen every month:
- The Moon’s orbital plane is inclined by about $5^{\u00b0}$ to the ecliptic; even when Sun–Moon–Earth are aligned for New or Full Moon, the Moon might be above or below the ecliptic plane and miss the shadow on Earth (or on the Moon).
A practical visual analogy used in lecture:
- Think of the Moon dipping above and below a pond (the ecliptic plane). Only when the Moon’s position aligns with the Sun as it crosses that plane do eclipses occur.
Additional note on science literacy:
- Flat-Earth arguments sometimes arise; the lecture emphasizes the evidence for a spherical Earth via shadow geometry and GPS evidence.

The Night Sky: Planets

A classic visual: a rare alignment showing multiple planets in a small sky region (e.g., the 2002 alignment of Mercury, Venus, Mars, Jupiter, Saturn).
Individual planets (as observed from Earth):
- Mercury: very close to the Sun; difficult to see; best observed near sunrise or sunset.
- Venus: often visible as the “morning star” or the “evening star,” due to its interior orbit between the Earth and the Sun.
- Mars: currently visible after sunset with a noticeable reddish hue.
- Jupiter: very bright after the Moon; a prominent “star” in the night sky.
- Saturn: visible at night; notable for its rings when sky conditions permit.
The lecture suggests that on Thursdays the discussion will continue with planetary motion and why planetary motion is complex to understand.

Quick Recap and What’s Next

Eclipses require two conditions: a Full Moon for Lunar eclipses or a New Moon for Solar eclipses; and near a node where the Moon crosses the ecliptic plane.
The Moon’s orbit tilt relative to the ecliptic (~ $5^{\u00b0}$ ) explains why eclipses don’t occur every month.
Two eclipse seasons occur each year; the Saros cycle (~ $18\text{ years}$ ) helps predict eclipses.
The northern sky’s navigational beacon, Polaris, is a temporary anchor due to axial precession; its status as the North Star will shift over millennia.
The Moon’s phases are a consequence of the Sun–Moon–Earth geometry and the Moon’s orbital motion (~ $27.3\text{ days}$ sidereal; ~ $29.5\text{ days}$ synodic).
The difference between the near/far sides of the Moon is a matter of perspective; both sides receive sunlight; the near side simply faces Earth.
The next topics ahead include deeper exploration of planetary motion and the dynamics that govern the orbits of the planets.

References to Important Dates, Numbers, and Concepts (for quick study)

Axial tilt: $23.5^{\circ}$
Precession period: $2.6\times 10^{4}\ \text{years}$
Moon–Earth distance (as stated in lecture): approximately $3.8\times 10^{4}\ \text{km}$
Moon orbital periods:
- Sidereal month: $27.3\ \text{days}$
- Synodic month: $29.5\ \text{days}$
Moon–Sun–Earth angular relationship in eclipses: alignment occurs near the nodes; orbital tilt relative to ecliptic: $5^{\circ}$
Ecliptic and nodes: eclipse seasons occur roughly every six months; approx. two per year
Saros cycle: roughly $18\text{ years}$
Major dates mentioned: autumnal equinox around September 22; summer solstice around June 21; winter solstice around December 21; next US solar eclipse around 2045; partial lunar eclipse in Minneapolis in 2026