The night is always darkest before the dawn

I've been spending a lot of time in the dark lately, but at least I live at 40 degrees north, not 70 degrees north in Tromso, Norway. This is a picture of a typical afternoon during the winter- although the sun isn't above the horizon, it illuminates the sky to a dark blue hue. (Image Credit: Osopolar (Own work), via Wikimedia Commons)

I've been spending a lot of time in the dark lately, but at least I live at 40 degrees north, not 70 degrees north in Tromso, Norway. This is a picture of a typical afternoon during the winter- although the sun isn't above the horizon, it illuminates the sky to a dark blue hue.

(Image Credit: Osopolar (Own work), via Wikimedia Commons)

I went to bed last night at 7 am. When I woke up this afternoon at 2 pm, I read an article that was simultaneously depressing and uplifting. And don't worry, I'm going to tell you all about it. But first, why the heck is my sleep schedule so disturbed? I was observing galaxies during the second half of the night (12-7am) and I'll be observing again tonight during the same time slot. It's great on one hand because I get the building all to myself, so, naturally, I play a lot of really loud Taylor Swift. But on the other hand, I feel like I haven't seen the sun in days.

But the Earth's rotation around the sun is also working against my internal clock. And that's where this article comes in. According to this news that I read at 2 pm right before the sun set, tonight is the longest night of the year AND the longest night ever with the exception of the winter solstice in 1912. Whoa - what's going on?

How could tonight be one of the longest nights in the 4.5 billion year history of Earth?

There's a couple of factors in this game.

First, the seasons. This is the longest night of the year for the northern hemisphere (night of December 21st, morning of December 22nd). This happens because of the tilt of the Earth as it orbits the sun. A very common misconception (that most graduating Harvard seniors believe) is that the seasons are caused by Earth being closer to the sun in summer and farther away in winter. Not only is this incorrect, but in my opinion it excludes everyone who lives south of the equator. Due to the tilt of the Earth, our friends down south are actually experiencing summer right now. Check out this infographic that demonstrates the illumination of the Earth during various seasons.

The tilt of the Earth causes more direct sunlight (hence more Energy) to fall on the northern hemisphere during the summer and vice versa. (Image Credit: Tom Ruen, Full Sky Observatory) 

The tilt of the Earth causes more direct sunlight (hence more Energy) to fall on the northern hemisphere during the summer and vice versa.

(Image Credit: Tom Ruen, Full Sky Observatory) 

The technical name for this tug-of-war is tidal braking. (Image Credit: AndrewBuck (Own work), via Wikimedia Commons)

The technical name for this tug-of-war is tidal braking.

(Image Credit: AndrewBuck (Own work), via Wikimedia Commons)

Second, gravity. Alright, blah blah blah so yeah it makes sense that this is the longest night of the year for the northern hemisphere. But why is this one of the longest nights EVER? The moon is torquing down the Earth's rotation slowly. It turns out that Newton was right - for every action, there's an equal and opposite reaction. This means that yes, the Earth is more massive than the moon, but the smaller moon exerts an equal force back on Earth. This force is evident in the Earth's tides. 

A great way to make this concept more intuitive is to imagine the massive amount of water the moon is dragging to create the tides. The earth is still rotating about it's axis, so there's this gigantic tug of war between the Earth attempting to drag it's oceans with rotation while the moon tries to hold them back. This competition works to slow down the Earth very slowly; between 15 millionths and 25 millionths of a second are added to the standard day yearly. But this is still a measurable difference.

Third, climate change. It turns out that releasing a bunch of warming agents into our atmosphere melts the ice on the poles. And when you melt ice on the poles this massive amount of water is redistributed around the equator. So the Earth has gained some love handles around the Equator. 

This explains why the longest night is not this year but instead back in 1912 before we started melting the ice at the poles. Having more mass around the equator actually makes the earth rotate slightly faster. 

So all in all, these three main factors influence why tonight is a VERY long night, but not the longest. For me, I'm excited because I get more time to observe galaxies without the sun interfering. I'll just have to go take a bunch of vitamin D supplements. But don't worry - from here on out the night will only get shorter as we march on towards summer. 

 

 

/