Today, September 23, is the spring equinox. The spring equinox marks the real astronomical start of spring. From today onwards the days are longer than the nights! But just what does that really mean?
The most commonly known definition of the equinox is when the day is the same length as the night. This occurs twice per year, once in September and once in March. In practice the date known as the equinox does not necessarily have exactly the same length of day and night. It depends on your location, the level of the horizon and other factors. The sun’s light is actually refracted by the atmosphere so it can appear above the horizon when it actually isn’t!
A more correct scientific definition of the equinox is the date and time when the centre of the sun is precisely above the equator. This year this occurs today(September 23) at 12:29 PM Australian Eastern Standard Time. Because the sun is a sphere it actually takes the sun around 33 hours to cross the equator in it’s entirety. The sun will reach it’s farthest point southward at the summer solstice which will be on December 21, when the sun will be directly over the Tropic of Capricorn.
Of course the sun isn’t actually moving south. The earth goes around the sun, not the other way around. The Earth isn’t changing it’s tilt either. So what’s actually happening?
The Earth spins on it’s axis every 24 hours, like a spinning top. The points where the axis enters and exits the Earth surface are known as the north and south poles. But this axis is not vertical relative to Earth’s orbit around the sun. In actual fact it is tilted 23.4° off vertical.
This never changes as the Earth goes around the sun. Consequently on the solstice in June the north pole is most inclined towards the sun. On the solstice in December the south pole is most inclined towards the sun. On the equinox neither the north or south poles is inclined toward the sun.
Another way of thinking about the equinox is by looking at the solar terminator. The solar terminator is the line that divides day and night. Normally the solar terminator is at an angle to the equator. On the equinox the solar terminator is exactly perpendicular to the equator. The picture below showing the solar terminators on the equinoxes and solstices in 2004 illustrates this well.
So that’s a basic explanation of what the equinox is and what it means. You can find out more technical information about the equinox on Wikipedia. You can track the sunrise time, sunset time and the length of the day on my Solar Day Calendar, as seen in the first picture. You can also see the equinoxes and solstices shown on the chart.