Autumnal Equinox

This past September 23rd, the northern hemisphere experienced the beginning of our fall season.

In terms of astronomy, what does this mean?

In the sky, if we could see the ecliptic, the path the Sun travels on as it appears to cross the sky, the point of the autumnal equinox occurs when the Sun crosses the celestial equator as it travels south on the ecliptic. This marks the beginning of autumn for all of us in the northern hemisphere, and the beginning of the spring for those in the southern hemisphere.

Another way to look at it, is that the Sun is directly overhead at the equator, which means that the Earth itself is tilted in the plane perpendicular to the Sun. Neither the northern hemisphere or the southern hemisphere is tilted towards the Sun. But at this point, the southern hemisphere will begin to receive more sunlight as the southern hemisphere is beginning to be tilted more towards the Sun.

Why Is the Sky Blue?

Believe it or not, we've actually discussed this before. When we've talked about reflection nebulae, we mentioned how the blue light is reflected towards our line of sight. In the same way, the sky is blue.

Remember, that light is composed of all colors of light. When light from the Sun enters the atmosphere, the dust in the sky easily reflects the blue wavelengths because they are shorter. The longer wavelengths (red, orange, yellow) pass right by the dust particles. The blue light is dispersed over the whole sky and when we look at the sky, we see blue. Of course, when we look at the Sun (which you should never do without proper eye protection), the Sun is yellowish (more yellow light in the Sun's spectrum than orange or red.

However, at sunrise or sunset, the sky around the Sun is orangish-red. Why is this? There are two reasons:

1.      The blue light is scattering away from our line of sight so all we see from the Sun are reds and oranges.

2.      The amount of atmosphere the light travels through to reach our eyes is more at sunrise and sunset than when the Sun is directly overhead. More dust (and pollution) for the blue light to interact with and be reflected away from us.



 

Sidereal Day Vs. Synodic Day

I’ve talked here before about sidereal days and synodic days. What exactly is the difference between the two?

It all comes down to one thing: the object that you are using as a reference. For example, the sidereal day of the Earth is equal to 23 hours, 56 minutes, and 4 seconds. The sidereal day uses a distant star (which is basically any star that is not the Sun) to measure against. You would determine how long a star would take to reach the same location in the sky on consecutive days. Another way to look at it, is the length of time it takes the Earth to rotate 360°.

However, the synodic day is 24 hours. The best way to think of the synodic day is to just determine the length of time between two consecutive noons. A synodic day means the Earth rotates more than 360° to get the Sun in the same location in the sky. The reason this happens is because as the Earth rotates on its axis, it is also moving around the Sun. Granted, it is only moving a little less than a degree in its orbit (since the Earth year is about 365.25 days), but that is long enough that the Earth has to rotate just a tad more to get the Sun back to noon.

 

Seasons on Earth

Recently, everyone on Earth went from one season to the next. In the northern hemisphere, we went from winter to spring and in the southern hemisphere, summer gave way to the fall (autumn). How exactly are the seasons defined?


Just this past Friday, the northern hemisphere experienced the vernal equinox, while in the southern hemisphere, the autumnal equinox occurred. We already know why they call these the equinoxes as they were explained in the previous posts. However, why does the northern hemisphere experience one season, while the southern hemisphere experiences the opposite (spring-fall and summer-winter)? It all has to do with the Earth's axial tilt.


The Earth rotates on its axis once a day. Most everyone knows that. However, the rotational axis of the Earth is tilted with respect to its orbital axis by about 23.5º.

So what does this mean?


When one half of the Earth is tilted towards the Sun, that half will receive more direct sunlight. The other half will not. It is like when you take a flashlight and aim it at a wall. If you keep the flashlight parallel to the wall, the light circle is tight and compact, if you tilt the flashlight away from the wall, the light circle becomes elongated. The same amount of light is hitting the wall, but in a larger area.

 

Another interesting thing about the seasons is that the time of year they occur change over time. However, none of us will be alive when January will be summertime in the northern hemisphere. This will occur in about 13,000 years. If you can wait until 28,500 CE (common era, formerly known as AD), April will be the middle of summer for the northern hemisphere. This is because the Earth wobbles on its axis, causing precession of the equinoxes (which in turn, also causes the solstices to precess). Currently, the summer solstice occurs when the Sun is in the constellation Gemini, but will someday be in the constellation Sagittarius.