At 23:21 UT (19:21 PM Eastern Standard Time), spring will officially be here for the northern hemisphere! It's been nice spring weather here for the past couple of days, but it's nice to have it be official. Although, I guess some people argue that it's really the middle of spring. I suppose both could be right, it's a matter of perspective and seasons are loosely based anyways. Wintery days can occur in Fall or Spring, and Fall can have Summer or Wintery days, and so forth. It tends to be more of an 'I know it when I see it' concept.
That said, what makes the Vernal Equinox special is that it is a midpoint in the Sun's journey across the sky. The Sun appears to travel against the sky throughout the year due to the Earth's tilt as it orbits the sun. On the equinox's (there is also the autumnal equinox in September), the sun sits over the equator, day and night are roughly the same length (12 hours) all over the Earth.
The term equinox means equal day and night. Ver is latin for spring, and autumnus is autumn, so these terms have a general bias for the northern hemisphere. Down below the equator, I'm sure you guys are getting ready for your colder months, since this is in reverse for you.
Also, I suppose I should dispel some incorrect notions about seasons. I have heard people tell me before that the reason for the seasons was because the Earth got closer and further away from the Sun due to Earth's non-circular orbit, or because the tilt of the Earth made one hemisphere closer then the other. It should be noted that distance from the Sun has nothing to do with the seasons, the temperature difference really is not that huge over Earth's orbit. In fact, during winter in the northern hemisphere, we are closer to the Sun!
The Earth's tilt is the reason for the seasons. And not because one hemisphere is closer to the Sun then the other, but because of it's angle in the sky. When you are in summer, your hemisphere of the Earth is tilted towards the Sun, meaning it is high in the sky with plenty of direct sunlight. When it is winter, you're tilted away from the Sun, it is low in the sky and there isn't a lot of direct sunlight, more of it reflected by the atmosphere.
Here's an infograph from NASA:
|How Seasons Work|
Another interesting fact about the Earth's tilt is that it's cause is thought to be the same collision that created our moon!
That's all I have for winter, I'll have a fresh spring post tomorrow. I have a wealth of information that I learned from my visit to the Applied Physics Lab at John's Hopkins. I'll probably do an over view post tomorrow and might have a couple posts over the next week to share the information.