June 21st is an important day this year. Not only is it the summer solstice (that is to say, the longest day of the year in the northern hemisphere), but it is also one of the longest days ever in the history of the Earth. Not only is it one of the longest days ever, but it’s Father’s day!
My dad inspired me to become a scientist and astronomer. He is one of the most curious people I know; in fact, I guarantee that he will be one of the first people to read this article. Back when I lived in a suburb of Seattle filled with light pollution, he would enthusiastically break out his refracting telescope. From the end of our driveway, pointing away from that damn streetlight that would never turn off, we’d gaze upon Saturn and Jupiter.
Fast forward a decade or so and I’m an over-caffeinated grad student anxiously awaiting the setting of the sun. Last week, I had an observing night on a 3.5 meter telescope down in New Mexico. I observe remotely from my office in Colorado (while blasting Taylor Swift of course), and where usually I would have at least five of six hours of darkness in a half-night to observe my favorite galaxies and black holes, now I was reduced to less than three. Why was I experiencing this time crunch? Astronomers have to wait for the Sun to set to see these extremely faint objects, and since we’re approaching the summer solstice, I’m losing my coveted nighttime rapidly.
However, it turns out that if I were observing centuries ago, I would actually have a couple fewer milliseconds of time to observe my galaxies. The Earth’s rotation is slowing down ever so gradually, contributing to the fact that this Father’s day is one of the longest days ever. Whoa – what’s going on?
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How could Sunday be one of the longest days in the 4.5 billion year history of Earth?
There’s a couple of factors in this game.
First, the seasons. Sunday is the longest day of the year for the northern hemisphere. This happens because of the tilt of the Earth as it orbits the Sun. A very common misconception is that the seasons are caused by Earth moving 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:
Second, gravity. Alright, so yeah it makes sense that this is the longest day of the year for the northern hemisphere. But why is this one of the longest days ever? The Moon’s gravity is tugging on the Earth, slowing down its rotation. 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 Moon exerts an equal force back on Earth. This force is evident in the way the Moon drags the water on Earth, a phenomenon we know as 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 its axis, so there’s this gigantic tug of war between the Earth attempting to drag its oceans along while the Moon tries to hold them back. This competition works to slow down the Earth’s rotation very gradually; 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 water is redistributed around the equator. So the Earth has gained some love handles. Meanwhile, the crust under the vacated ice actually springs back, adding some mass under the poles. Having more water mass around the equator but comparably more land mass in the poles actually makes the Earth rotate slightly faster.
This explains why the longest recorded day was not this year but instead back in 1912 before we started melting the ice at the poles.
Fourth, earthquakes. Earthquakes and other natural events such as temporary shifts in the polar ice caps can alter the rotation time of Earth on millisecond scales over yearly periods. This is an additional reason that this current solstice isn’t the longest period of daylight ever. This is also why it’s not a good idea to blindly claim that the current or upcoming year will set a new record for the longest days ever.
So all in all, these four main factors influence why today is a VERY long day, but not the longest. For me, I’m excited that everyone gets that millisecond or two of extra daylight to BBQ with their dad. I’ll just have to wait for a couple months until my coveted nighttime hours get longer as the seasons change. But don’t worry – from here on out the night will only get longer as we march on towards winter.