The space station watches the sun rise over the limb of the Earth. But how can you tell if this is the first sun rise of spring? Click to enlarge this image.
NASA
While walking to my usual coffee shop this morning, it was hard to believe March 1st had arrived. The temperature was a little over 3 degrees Celsius (37 F), hardly what you'd expect of spring.
I decided to be brave and take my hands out of my pockets and check my twitter feed. Some bright spark told me it was "like summer" in Ireland (thanks!), and someone else was asking if today was the first day of spring.
That got me thinking, but it surprised me a little that there was still confusion. The seasons can actually be tricky blighters, hard to define and pin down. For me, with a chilly and cloudy UK morning (warming my hands on a fresh cup of tea), I could really do with a morale boost and find out when spring actually starts.
What Is It That Makes the Seasons?
Well, it's all down to the Earth's orbit around the sun. Imagine the sun sitting at the center of a vast sheet of paper. All the planets, Earth included, orbit around the sun on this celestial sheet. It represents the plane of the solar system, with the official name of "the ecliptic."
For the Earth, this journey takes about one year and at the same time it is spinning on its axis, giving us the day that lasts a smidgen under 24 hours. This same rotational axis is tilted over by 23.5 degrees.
How does all this hook into the seasons? Well, it's the tilt of the Earth on its axis alternately pointing toward and away from the sun throughout the year that gives us the seasons.
As the Earth's northern hemisphere points toward the sun in June, the northern hemisphere enjoys summer while the southern hemisphere 'enjoys' winter. Just six months later when the Earth's southern hemisphere points toward the sun in December, the northern countries experience winter and the southern ones, summer.
The other seasons, spring and autumn, come in between as the hemispheres are warming up or cooling down.
Who Says it's Spring?
Trying to tie down the first day of spring proves a little trickier than just understanding the concept of the seasons and, depending on who you talk to, you will actually get two different answers.
A meteorologist will tell you that the seasons are based purely on average temperatures; temperate northern hemisphere dwellers will be basking in the increasing warmth of spring during March, April and May. The meteorologist then will say the first day of spring is usually on or around March 1st. That's awesome, it's today! I'm feeling warmer already.
Unfortunately though, if you talk to me (an astronomer) I will tell you something completely different. My view, and that of most other astronomers, is that the coming of spring is heralded by the Spring or Vernal Equinox (Vernal comes from Latin for "bloom") on March 20th or 21st. This may seem like a pretty random date but it is determined by the orbital movement of the Earth.
We've already seen that it's the tilt of the Earth's axis that gives us the phenomenon of the seasons but it's the Earth's arrival at a specific point in its orbit that determines the start of spring.
When the Ecliptic and Celestial Equator Cross
From the view point of the Earth, we can extend the equator onto the sky and call it the celestial equator which now depicts a circle all around the sky. We can also imagine that while the Earth orbits around the sun, from down here on the surface, it can look like the sun orbits around us. If we plot the position of the sun around the sky in a year, it will mark out a different circle in the sky which is the plane of the ecliptic of the solar system.
Because the ecliptic and celestial equator are tilted with respect to one other, there are two points in the sky where they cross and when the sun sits at either of these, we experience either one of the two equinoxes (literally translated to "equal night" in Latin). At this point, the length of day and night is approximately the same.
When the sun is at the point when it's heading north above the celestial equator, it marks the start of spring in the northern hemisphere and autumn in the southern hemisphere. This is the Spring Equinox. When it's heading south, in September, it marks the onset of autumn in the north and spring in the south.
So you see, there are really two answers to the question when is the first day of spring. A meteorologist will say March 1st, but an astronomer (and convention) will say March 20/21 (for southern hemisphere readers the start of spring will be either 1st September or 20/21 September respectively).
I guess I rather prefer the meteorological answer as it means spring has sprung, but in reality the astronomical definition is a much more accurate one and one that I think we should stick with.
But then again, I am biased.
comments ( )