We got a map of the new solar system from National Geographic that we have hung in my son's bedroom.

On one side it shows how long it takes each planet to revolve around the sun and rotate on its axis (what a "year" for each planet is and what a "day" is). I think someone got a phone call from National Geographic at GPL and they decided to have fun with them when they asked "how many hours in one Earth day" because according to this chart, there are 23.9 hours in one Earth Day.

Last I checked, it was almost precisely 24 hours. I remember a long time ago they had to add a fraction of a second at the end of a year. So I think our time pieces are very accurate about the length of a day. But I thought I would post this here to see if anyone has any special insight.

The Earth's *sidereal* day is 23h 56m 4s which is 23.93 hours. So rounding to 23.9 is fair enough.

see for example: http://scienceworld.wolfram.com/astronomy/SiderealDay.html

James

jamescanvin are absolutely right.Theres 24 hours between sunrise and sunset. But Earth rotates by 23 hours 56 minutes or "23.9 hours"

During the same day, Earth will move a bit in its orbit around the sun. And so the only way to check on Earths actual rotation are to measure the rise and setting of a star - therefore the word Sideral day.

Great explanations James and Myron. When you go to planet tables that give periods for a planet’s rotation, orbit about the sun, eccentricities, etc., do they (astronomers or astrophysicists) usually list sidereal or solar day? I'm not sure all tables indicate which they list. I'm now educated and forewarned.

Thank, Floyd

My favorite source is the fact sheets at the NSSDC.
http://nssdc.gsfc.nasa.gov/planetary/planetfact.html

--Emily

The leap seconds you are referring to aren't added every year. In fact, there has only been one added so far in the 21st century, and that was just under 12 months ago. These leap seconds are occasionally added to keep the exactly 24h civil clock we all live by in synch with the Earth's rotation. Technically, it is to keep a timescale called UT1 that follows the Earth's rotation within 0.9 seconds of the standard clock, also known as UTC or ("Zulu"). A group called the International Earth Rotation Service publishes a precisely measured difference between UT1 and UTC.
Here is an historical list of leap seconds.