Seasons Notes: Solstices, Equinoxes, Subsolar Point, and Circle of Illumination

Solstices, Equinoxes, Subsolar Point, and Day Length

  • What is a solstice?

    • June solstice: occurs around June 21. On that day the subsolar point is at the Tropic of Cancer (approximately 23.5exto23.5^ ext{o} N). The Northern Hemisphere receives more direct sun, leading to summer in the north and winter in the south.
    • December solstice: occurs around December 21. On that day the subsolar point is at the Tropic of Capricorn (approximately 23.5exto23.5^ ext{o} S). The Southern Hemisphere receives more direct sun, leading to summer in the south and winter in the north.
    • The subsolar point is the location on Earth where the Sun is directly overhead.

  • What is an equinox?

    • Equinoxes occur when the subsolar point hits the equator (latitude 0exto0^ ext{o}). There are two per year.
    • On equinox days, the sun is at the same altitude everywhere, yielding a day length of about twelve hours of daylight and twelve hours of night for all latitudes, including extreme latitudes such as Reykjavik (~66exto66^ ext{o} N).
    • In other words, the subsolar latitude δ = 0^ ext{o} on equinox days.

  • Subsolar point and latitudes to know

    • Tropic of Cancer: +23.5exto+23.5^ ext{o} (N)
    • Tropic of Capricorn: −23.5exto-23.5^ ext{o} (S)
    • Arctic Circle: approximately +66exto+66^ ext{o} (N)
    • Antarctic Circle: approximately −66exto-66^ ext{o} (S)
    • The subsolar point moves between the Tropics over the year, with a latitude range of roughly between Tropic of Cancer and Tropic of Capricorn.

  • Circle of illumination (terminator)

    • The circle of illumination is the dividing line between day and night on Earth; it is also known as the terminator.
    • Its position is constantly changing because the Earth is spinning, so the boundary between light and dark shifts as the planet rotates.
    • This moving boundary creates the day/night cycle for any given location.
    • The diagram or graph often highlights how, regardless of where you are, the axis of the Earth remains parallel to itself throughout the orbit, which drives the seasonal pattern.

  • Why seasons happen (the role of axis tilt and parallelism)

    • The Earth’s axis is tilted relative to its orbital plane around the Sun by about 23.5exto23.5^ ext{o}. This tilt causes different hemispheres to tilt toward or away from the Sun during the year.
    • The property of parallelism: the axis remains parallel to itself at all points in its orbit. This means that as the Earth travels around the Sun, one hemisphere can lean toward the Sun at one time of year and the other hemisphere leans toward the Sun at the opposite time.
    • On the June solstice, the Northern Hemisphere is tilted toward the Sun, receiving higher sun angles; on the December solstice, the Southern Hemisphere is tilted toward the Sun, receiving higher sun angles.
    • The tilt and the orbital geometry cause varying solar angles and day lengths, leading to seasons.

  • Day length and latitude (illustrative example)

    • On the June solstice, at a mid-latitude such as around 30exto30^ ext{o} ( ext{latitude}), about two-thirds of the line at that latitude is on the daylight side and one-third on the nighttime side. This yields a long day and short night.
    • The opposite occurs in the Southern Hemisphere during the Northern Hemisphere’s summer, with the corresponding latitude experiencing a long or short day depending on its position relative to the circle of illumination.
    • The exact day length depends on how much of the latitude line is on the daylight side of the Earth.

  • Key observational points from the visuals/text

    • On the June solstice, the subsolar point is at +23.5exto+23.5^ ext{o} (N) and the Arctic Circle experiences 24 hours of daylight; the Antarctic Circle experiences 24 hours of darkness.
    • On the December solstice, the subsolar point is at −23.5exto-23.5^ ext{o} (S) and the Arctic Circle experiences 24 hours of darkness; the Antarctic Circle experiences 24 hours of daylight.
    • On the equinoxes, the subsolar point is at the equator (0exto0^ ext{o} latitude), and the circle of illumination passes vertically around the globe, giving twelve hours of daylight and twelve hours of night everywhere.
    • The term “circle of illumination” is sometimes referred to as the “terminator.”

  • Practical implications and real-world relevance

    • The tilt and parallelism explain why different hemispheres have opposite seasons at the same time of year.
    • The variation in day length drives climate patterns, energy balance, and biological rhythms in different regions.
    • Polar regions experience extreme daylight/darkness cycles around solstices due to their proximity to the Arctic/Antarctic Circles.
    • The concept helps explain everyday observations, such as hotter summers in the NH when it is tilted toward the Sun and colder winters when it is tilted away.

  • Connections to foundational principles

    • Rotation causes the day/night cycle and the daily movement of the circle of illumination.
    • Axial tilt and orbital geometry around the Sun create seasonal variations in solar declination and solar altitude, which govern the intensity and duration of sunlight.
    • The concept of the subsolar point as the current point of direct overhead Sun links to solar declination and Earth’s geometry.

  • Quick notes for study

    • Read Chapter 2 before the next class; the lecturer indicates they will start covering it next session.
    • Remember the terminology: subsolar point, circle of illumination, and terminator.
    • Be able to explain why the equinox results in 12-hour days everywhere and why solstices mark the extremes of day length in respective hemispheres.

  • Metaphors and informal illustrations used

    • The circle of illumination is described as a boundary or shadow that moves across a fanciful map of the Earth, akin to a shadow on a rotating object (the student joked about a wooden piece and a shadow).
    • A map-like visualization helps connect the geometry (tilt and rotation) to the real-world pattern of daylight distribution.

  • Summary relationships to memorize

    • Solstice dates: around June 21 (Northern tilt toward Sun) and around December 21 (Southern tilt toward Sun).
    • Subsolar latitude at solstices: +23.5exto+23.5^ ext{o} N (June) and −23.5exto-23.5^ ext{o} S (December).
    • Equinox dates: twice a year; subsolar latitude 0exto0^ ext{o}; day length ~12 hours everywhere.
    • Subsolar point latitude range: between Tropic of Cancer and Tropic of Capricorn.
    • Polar circles: ~66exto66^ ext{o} N/S; 24 hours of daylight or darkness occur around solstices.

  • Closing reminder

    • The basic mechanism of seasons is rooted in the tilt of the Earth's axis and the constant tilt angle as the Earth orbits the Sun, with the subtended solar geometry producing hemispheric differences in sunlight and day length throughout the year.