The Earth’s journey around the Sun, a year-long elliptical orbit, and its constant spin on its axis are fundamental to our experience of day and night. However, the captivating change of seasons isn’t just about this orbit; it’s intrinsically linked to the Earth’s axial tilt. This tilt, the angle of our planet’s spin axis relative to its orbital plane, is the prime reason we experience summer, winter, spring, and fall.
When the Earth’s axis leans towards the Sun, a hemisphere basks in the warmth of summer. Conversely, when the axis tilts away, that same hemisphere experiences the chill of winter. This phenomenon arises from the consistent 23.5-degree tilt of our Earth. The North Pole never points directly at the Sun, but it reaches its closest point during the summer solstice and its furthest during the winter solstice. In the transitional periods of spring and autumn, the Earth’s axis is oriented at a 90-degree angle to the Sun, resulting in roughly equal day and night durations across the globe.
To grasp how this axial tilt influences our weather, consider a simple illustration. Imagine shining a flashlight directly onto a piece of paper. You’ll observe a concentrated circle of light. This circle represents the sunlight hitting the Earth directly. Now, gradually tilt the paper. The circle of light stretches into an ellipse, spreading the same amount of light over a larger area. This dilution of light demonstrates how the angle of sunlight affects its intensity.
Similarly, on Earth, when the sun is directly overhead, its rays strike the surface perpendicularly, concentrating energy and warmth. This is why midday sun feels hotter. However, when the sun is lower in the sky, due to the Earth’s tilt, sunlight is dispersed over a wider area. This oblique angle reduces the amount of energy absorbed per square centimeter of the Earth’s surface, leading to less heat. The Earth’s axial tilt ensures that different parts of the planet receive more direct sunlight at different times of the year.
For the Northern Hemisphere, the period around June 21st marks when the axis is most inclined towards the Sun, ushering in the summer solstice. Conversely, around December 21st, the axis tilts furthest away, bringing the winter solstice. The term “solstice,” derived from Latin, signifies “sun stands still,” referring to the point when the sun appears to stop moving northward or southward in the sky. The seasons are reversed for the Southern Hemisphere; when the North experiences summer, the South experiences winter, and vice versa.
Around March 21st and September 21st, the Earth reaches points in its orbit where its axis is at a 90-degree angle to the sun for both hemispheres. These dates are known as the Spring and Fall Equinoxes. “Equinox,” from Latin, means “equal night,” aptly describing the near-equal duration of day and night experienced worldwide during these periods.
Image showing the Earth
Alt text: Diagram illustrating Earth’s axial tilt at 23.5 degrees relative to its orbital plane, demonstrating how this tilt causes different hemispheres to receive varying amounts of direct sunlight throughout the year, leading to seasons.
Why Aren’t Day and Night Exactly 12 Hours Long on the Equinox?
While equinoxes are associated with equal day and night, the reality is slightly different. True equal day and night, meaning exactly 12 hours each, actually occur a few days before the March equinox and a few days after the September equinox. The precise dates for this balance vary depending on latitude.
On the equinox day, the geometric center of the Sun crosses the equator, positioning it above the horizon for 12 hours everywhere on Earth. However, the Sun isn’t a point source of light; it’s a disk. Sunrise is officially defined as the moment the leading edge of the Sun’s disk becomes visible, and sunset is when the trailing edge disappears. At these moments, the Sun’s center is already below the horizon.
Furthermore, atmospheric refraction plays a role. As sunlight enters the Earth’s atmosphere, it bends, causing the Sun to appear higher in the sky than its actual geometric position. This refraction effect means we see the top edge of the Sun a few minutes before its geometric edge rises above the horizon in the morning. Similarly, in the evening, the top edge remains visible for a few minutes after the geometric disk has set.
For locations near the equator, daylight consistently lasts slightly longer than nighttime throughout the year due to these effects. At higher latitudes in the Northern Hemisphere, equal day and night occur before the March equinox, and daylight hours remain longer than nighttime until after the September equinox. The opposite is true for the Southern Hemisphere, where equal day and night happen before the September equinox and after the March equinox.
Equinoxes and Solstices Dates Through 2030
The following table provides the dates and times for equinoxes and solstices up to 2030, listed in Eastern Time. For Central Time, subtract one hour. (Source: U.S. Naval Observatory)
| Year | Spring Equinox | Summer Solstice | Fall Equinox | Winter Solstice |
|---|---|---|---|---|
| 2025 | Mar 20 — 5:01am | June 20 — 10:42pm | Sept 22 — 2:19pm | Dec 21 — 10:03am |
| 2026 | Mar 20 — 10:46am | June 21 — 4:24am | Sept 22 — 8:05pm | Dec 21 — 3:50pm |
| 2027 | Mar 20 — 4:25pm | June 21 — 10:11am | Sept 23 — 2:02am | Dec 21 — 9:42pm |
| 2028 | Mar 19 — 10:17pm | June 20 — 4:02pm | Sept 22 — 7:45am | Dec 21 — 3:19am |
| 2029 | Mar 20 — 4:02am | June 20 — 9:48pm | Sept 22 — 1:38pm | Dec 21 — 9:14am |
| 2030 | Mar 20 — 9:52am | June 21 — 3:31am | Sept 22 — 7:27pm | Dec 21 — 3:09pm |
Alt text: Humorous image depicting a chicken looking at a standing egg, referencing the myth that eggs can only be balanced on their end during the Spring Equinox.
The Equinox Egg Balancing Myth: Fact or Fiction?
The popular myth that you can only stand an egg on its end during the Spring Equinox is, in fact, a myth. While it is indeed possible to balance an egg on its end, it can be done on any day of the year, given enough patience and a steady hand. This notion resurfaces annually around the equinoxes, fueled by misunderstanding.
The myth suggests a special gravitational alignment during the equinoxes facilitates egg balancing. Some believe a balance between the Sun’s and Earth’s gravitational pulls creates this phenomenon. However, the Moon exerts a far more significant gravitational influence on Earth than the Sun, evident in ocean tides. While the Moon’s effects vary across equinoxes, the dominant gravitational force acting on a standing egg is simply the Earth’s gravity pulling the egg down onto the surface it rests upon.
To debunk this myth yourself, take a fresh, uncooked egg and gently place its larger end on a flat surface. Allow the egg’s internal fluids to settle, then carefully attempt to find its balance point. With persistence, you’ll discover that you can balance an egg upright any day, proving it’s about physics and patience, not seasonal alignment.
Learn more about the difference between astronomical and climatological seasons.
