# Simple riddle involving time travel

This is a simple one, but it's BTTF (Back to the Future) day, so I figured it was appropriate:

What goes a minute in four seconds?

• A really bad clock? Four regular clocks? :P
– Deusovi
Commented Oct 21, 2015 at 14:25
• Technical: <blahblah>relativity< /blahblah> Something to do with time dilation, time-travel, and relative velocities. Commented Oct 21, 2015 at 14:30
• The TARDIS? The Doctor? Commented Oct 21, 2015 at 14:42
• Something travelling at ~900 knots. Commented Oct 21, 2015 at 14:43
• "simple" and "time travel" are not known for going together well. Commented Oct 21, 2015 at 20:11

The

Earth rotates through one minute of arc every 4 seconds.

• Got it. I figured it'd be a pretty simple one, but it was fun to think about. Commented Oct 21, 2015 at 14:45
• Hmm... The title was somewhat misleading then. Where's the time-travel? Commented Oct 21, 2015 at 14:46
• @chaslyfromUK : That was intentional misdirection. However, the other answer I was intending and would have accepted was the Sun, which travels across the sky over a period of time. So: time, travel. Just not exactly time travel. Commented Oct 21, 2015 at 14:49
• @chaslyfromUK: well.. one travels four second ahead in time in that four second time lapse. Nothing fancy, one sec per sec speed, but it is time traveling :-) But you are correct. Too.
– user10179
Commented Oct 21, 2015 at 18:53
• @DarrelHoffman - By the way I liked the riddle and upvoted it. I just don't personally think misdirection is a good idea. I tried it myself on one of my first few questions and it just caused annoyance. I believe that, with a little extra thought, it is possible to make the title and clues accurate but not obvious. Commented Oct 21, 2015 at 19:04

The hour hand on a 24-hour clock.

There are 360 degrees in a circle and 60 minutes in a degree, so a minute is 1/360/60 = 1/21600 of a circle. The hour hand moves 1/24/60/60 = 1/86400 of a circle each second, so in 4 seconds it moves 4/86400 = 1/21600.

Note: If you say "earth", make sure to qualify that you are talking about its rotation with respect to the sun, since its rotation with respect to the stars is about 1/365 off of one full rotation each day.

• +1: Can't deny that this would also be correct, though not the intended answer. (I was thinking of the sun's position in the sky, which takes the relative rotation into account, but was planning to accept "earth" as well, since it depends on your point of reference.) Commented Oct 21, 2015 at 18:01
• Your equation isn't balanced, just a typo you might want to fix. 4/21600 != 1/21600. :) Commented Oct 21, 2015 at 20:04

There is no time travel involved.

The fastest cars in the world (not counting drag racers)

Because... (The math is pretty rough but I think it shows I'm on the right track)

"Minute" means "minute of arc" and not a unit of time
Diameter of Earth = 7,917.5 miles
Perimeter of Earth = 24,873.6 miles (I know it's not really a sphere)
One degree of arc = 69.1 miles
One minute of arc = 1.15 miles
Top speed of the Venom GT = 270 mph
Distance traveled by Venom GT in 15 seconds = 1.13 miles

• Ahhhhh! OP edited as I was answering. I was on the right track, though, for the original time of 15 seconds. Commented Oct 21, 2015 at 14:45
• Sorry about that. My original mistake was based on a rewording of the puzzle: "What goes fifteen minutes in a minute?" Which is the same thing, just multiplied. Commented Oct 21, 2015 at 14:46
• @EngineerToast, StackExchange sites: where you post as quickly as possible regardless of quality before ninja editing a correct answer (for the timestamp!) Commented Oct 21, 2015 at 16:07
• If you only care about longitude, any vehicle traveling slowly around the south pole could accomplish the same thing :) Commented Oct 22, 2015 at 15:59
• @acbabis Fair point, since the minute of arc here is in reference to the Earth's rotation. Commented Oct 22, 2015 at 17:02