“Solstice Simplified: A Stellar Astronomer Breaks it Down”

Astronomy and the Solstice: Understanding the Long and Short of Days, Years, and Seasons

By Jonti Horner, Professor of Astrophysics, University of Southern Queensland

Published: June 21, 2023 9.26am BST

Happy solstice everyone! As we approach the mid-year solstice, it is important to understand the significance of this celestial event and its impact on our day-to-day lives. The solstice, whether it be the winter solstice in the southern hemisphere or the summer solstice in the northern hemisphere, is a result of the Earth’s orbit around the Sun and the tilt of its axis. In this article, we will explore the mechanics of our planet’s motion, the cause of the seasons, and the significance of the solstices.

Earth’s Orbital Motion and the Changing Seasons

Earth is a moving platform, orbiting the Sun in a little more than 365 days. Despite our incredible orbital speed, we do not feel this motion. Instead, it appears to us as though the Sun is moving through the year. If we were to remove Earth’s atmosphere and observe the background stars simultaneously with the Sun, we would notice that the distant stars rise and set every 23 hours, 56 minutes, and 4 seconds – the true rotation period of Earth. However, the Sun rises and sets approximately every 24 hours, which makes the “solar day” 3 minutes and 56 seconds longer than Earth’s true rotation period.

This discrepancy in timing is due to the Sun’s apparent motion against the background stars. If we were to observe the Sun from our imaginary airless Earth, we would see it gradually sliding through the constellations of the zodiac, completing one full lap of the sky in one year. However, the Earth’s axial tilt of about 23.5 degrees adds complexity to this motion.

As we orbit the Sun, our planet alternately tilts one hemisphere towards the Sun and then away again. This tilt is the cause of the seasons. When a hemisphere is tilted towards the Sun, it experiences summer with long days and the noonday Sun high in the sky. Conversely, when a hemisphere is tilted away from the Sun, it experiences winter with shorter days, a low noonday Sun, and a chill in the air. The Sun also gradually drifts north and south between these extremes, reaching its highest point 23.5 degrees north or south of the Equator during the solstices.

The Significance of the Solstices

The solstices mark the points at which the Sun is either the farthest north in the sky (as we experience today) or at its most southerly location. During the winter solstice in the southern hemisphere, the noonday Sun appears lowest in the sky, resulting in the shortest period of daylight of the calendar year. In contrast, the summer solstice in the northern hemisphere places the noonday Sun high in the sky, resulting in the longest period of daylight. Six months from now, on December 22, we will experience the other solstice, marking the point at which the Sun is at its most southerly point in the sky. This will bring the longest day for the southern hemisphere and the shortest day for the northern hemisphere.

Understanding the solstices allows us to predict the Sun’s movement and the changing lengths of days throughout the year. Many websites provide information on sunrise and sunset times for specific locations, enabling individuals to plan and appreciate the different patterns of daylight throughout the year.

Defining the Seasons: Climate or Cosmology?

The solstices hold broader significance beyond astronomical events. For astronomers and many people worldwide, they mark the change of seasons. However, our understanding of seasons differs depending on our perspectives and experiences. In Australia, for example, most people associate seasons with the first day of the months of March, June, September, and December, which aligns better with how our climate behaves.

In a simple universe, one would expect the longest day to be the hottest, and the shortest day to be the coldest. However, the complexity of our planet’s atmosphere, ground, and oceans disrupts this expectation. These elements take time to heat up and cool down, which results in a delay between solstice and the warmest/coldest days. For many places, the warmest time of the year comes a few weeks after midsummer, while the coldest time in winter is usually a few weeks after midwinter. Our concept of summer and winter is built around these delayed effects rather than the astronomer‘s definition.

There’s Always Another Secret

Before concluding, it is worth noting that the seasons’ lengths are not equal throughout the year. Although we may perceive each season as lasting three months, a closer examination reveals some surprising variations. Some months, such as February, have fewer days than others. When we add up the days in each astronomical season, we find that the southern hemisphere summer (northern winter) lasts just 89 days while the southern winter (northern summer) is almost 94 days long.

These variations in season length are a result of Earth’s orbit around the Sun. As our distance from the Sun changes slightly during the year, Earth’s rate of orbital motion varies. We move faster when we are closer to the Sun and slower when we are more distant. For instance, on July 7, Earth will reach its farthest point from the Sun, known as “aphelion,” when we will be more than 152 million kilometers away. However, on January 3, 2024, we will be at our closest point to the Sun, known as “perihelion,” just over 147 million kilometers away.

This delightful complexity is one of the beauties of astronomy. As we delve deeper into our understanding of celestial mechanics, we uncover more exquisite secrets. So, as we celebrate the solstice, let us embrace the wonders of our universe and look forward to another 93 days of winter!