Sunsets are Quite Interesting

Bad Astronomy
The entire universe in blog form
Nov. 20 2011 7:14 AM

Sunsets are Quite Interesting

There's a wonderful comedic quiz show in the UK called "QI" -- for "Quite Interesting" -- which is hosted by none other than Stephen Fry. The participants are comedians, and they're asked questions ranging over just about every topic you can imagine. The BBC recently uploaded a clip about which alert BA Bloggee Brett Warburton informed me. In it, Fry shows the contestants a video of the Sun setting, and asks them to ring in when they think the Sun has completely set. Here's the clip:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


This is, in fact, correct! The Earth's air bends the image of the Sun upward, so we can still see the Sun even though it is physically below the horizon. If we didn't have air, daytime would be shorter. In fact, this effect works for sunrise as well, so we see the Sun rise before it's physically cleared the horizon.

And Stephen was correct in the amount too; the light is bent upward by just about the same size as the Sun, so when the lower limb of the Sun just kisses the horizon it's actually already set.

But it's a bit more complicated, of course. The amount of bending changes as the Sun dips lower, because we're seeing through more air the closer to the horizon we look. I've written about this before. In fact, I found a nice graph from NOAA showing the amount of bending:

The amount a ray of light is bent is shown on the vertical axis, and the distance from the horizon on the horizontal. The units are degrees, with 360° in a circle, of course, and the Sun's size in the sky is about 0.5°. Reading off the graph, you can see that right on the horizon, a ray of light is bent upward by about a half degree. In fact, the light from the top of the Sun is bent less than the light from the bottom, so this effectively pushes the bottom of the Sun up toward the top, squishing it! You've probably seen countless pictures of the Sun looking squashed on the horizon (the Moon, too -- I just posted about that yesterday, in fact!). Well, that's why.

Now, what the comic was saying about the roads in New Zealand didn't make much sense to me; the Sun gets low to the horizon everywhere at some point during the day (well, except maybe for the poles during their local summer, but they don't have well-traveled roads anyway), so glaring reflections are always a problem.

I am endlessly fascinated by phenomena like this in the sky, and I'm pleased that "QI" was able to explain it to so many people. Even if one of the comics didn't like it. But there you go: the Universe is under no obligation to make us like it. It does what it does, and you might as well enjoy it when you can.

Image credits: NOAA; NASA

Related posts:



Forget Oculus Rift

This $25 cardboard box turns your phone into an incredibly fun virtual reality experience.

The Congressional Republican Digging Through Scientists’ Grant Proposals

Renée Zellweger's New Face Is Too Real

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?

Whole Foods Is Desperate for Customers to Feel Warm and Fuzzy Again

The XX Factor

I’m 25. I Have $250.03.

My doctors want me to freeze my eggs.

The XX Factor
Oct. 20 2014 6:17 PM I’m 25. I Have $250.03. My doctors want me to freeze my eggs.

Smash and Grab

Will competitive Senate contests in Kansas and South Dakota lead to more late-breaking races in future elections?

George Tiller’s Murderer Threatens Another Abortion Provider, Claims Free Speech

These Companies in Japan Are More Than 1,000 Years Old

  News & Politics
The World
Oct. 21 2014 3:13 PM Why Countries Make Human Rights Pledges They Have No Intention of Honoring
Oct. 21 2014 1:12 PM The Global Millionaires Club Is Booming and Losing Its Exclusivity
The Vault
Oct. 21 2014 2:23 PM A Data-Packed Map of American Immigration in 1903
  Double X
The XX Factor
Oct. 21 2014 3:03 PM Renée Zellweger's New Face Is Too Real
  Slate Plus
Behind the Scenes
Oct. 21 2014 1:02 PM Where Are Slate Plus Members From? This Weird Cartogram Explains. A weird-looking cartogram of Slate Plus memberships by state.
Brow Beat
Oct. 21 2014 1:47 PM The Best Way to Fry an Egg
Oct. 21 2014 10:43 AM Social Networking Didn’t Start at Harvard It really began at a girls’ reform school.
  Health & Science
Climate Desk
Oct. 21 2014 11:53 AM Taking Research for Granted Texas Republican Lamar Smith continues his crusade against independence in science.
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.