h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Of course the length of Earth's sidereal day, approximately 23.9345 hours (or 23 hours, 56 minutes, and 4.1 seconds), would be the more useful figure. The fact that there are 24.0000 sidereal hours in the day is more of a definitional than informational nature.

I think they should have both entries, the human working day and the actual day.

if 23 hours, 56 minutes, and 4.1 seconds is a sidreal day, would it mean that we are actually running short by a day every year, which means we add 4 days every leap year?
Explain this to me please, I am a novice in this field.
Thank you.

For the human life, the 24 hours day is the most important one. This is the time between two sunsets.

The sidereal day is the time of one Earth rotation. It's different because Earth does not only rotate but also move around the sun. The 'year' is effectively another day. Imagine an Earth that isn't spinning at all: Then one year would mean one day. However, this 'year-day' is working in the opposite direction: The movement of the sun in the sky would be retrograde.

This 'additional' day moves forward very slowly and -- because it's retrograde -- makes our ordinary day to come to an end later than one rotation, approximately four minutes later. This yields our 24 hours day.

The leap year is something very different. Because the year is not a whole-numbered multiple of a 24-hours day, we have to introduce leap years to ensure that spring begins on 21st march every year, even in 1000 years in the future.

(just to complicate things, my geophysicist boyfriend points out that the Earth does not rotate on its axis in a uniform manner. Hence the occasional 'leap seconds' that one gets at the end of the year)

(just to complicate things, my geophysicist boyfriend points out that the Earth does not rotate on its axis in a uniform manner. Hence the occasional 'leap seconds' that one gets at the end of the year)

(Does the Earth shake around while it's on its axis? Do the leap seconds all add up to an extra day once every 10 000 years or so?)

'Does the Earth shake around while it's on its axis?' -- yes, this is called 'precession' and 'nutation'.

'Do the leap seconds all add up to an extra day once every 10 000 years or so?' -- First, leap seconds have another cause, namely that Earth's rotation speed is not constant. It varies due to several things, e.g. tidal effects or the current snow layer thickness, and mass movements in the core of the Earth.

I don't know how leap seconds add up. Nobody knows exactly, because the underlying effects are by and large unpredictable. At the moment the difference between UTC (your watch) and TAI (atom clock) increases by 2 seconds per 3 years. This would mean a day in 130,000 years. But I expect it to be much longer, because some of the effects are periodic.

The most important long term effect is the tidal retardation caused by the moon. Eventually one day would be one month long. But not if it goes as slowly as now: The day gets one second longer every 60,000 years.

The Earth does shake around on its axis. What is the difference between precession and nutation?

The leap seconds are basically ignored, I don't think it makes a difference. I got a globe for my birthday. Are there really mass movements in the core of the Earth?

Leap seconds are different to leap years; they are caused by different things. What do UTC and TAI stand for? (What is an atom clock?). If you don't mind me asking, how do you know all this? Is it part of your career?

Tidal retartation is such a weird name for a term to me. I don't understand how one day can be one month long. Thank you for your help.

The difference between precession and nutation is rather big but difficult to explain. Maybe you look for Internet pages containing both words and hope for a graphics. I have a too poor net connection for that.

Shortly, precession is a very slow rotation of the north-south axis of Earth around another axis. (The *axis* rotates here, not Earth.) I think one rotation lasts 25,000 years. Nutation is some sort of nodding of the axis, up and down. Both happens simultaneously.

There are indeed mass movements within Earth, and for leap seconds they are significant.

UTC -- "Universal Time Coordinated" This is the time on your watch.

TAI -- "Time Atomic International" This is the continuous regular scientific time of atom clocks, which are the most accurate clocks on Earth.

I'm a physicist, but I know this because I'm a (theoretical) amateur astronomer.

Tidal retardation means that the tides take rotation energy away from Earth, so it rotates more and more slowly. Eventually one rotation will last 30 days, this is one month. Then one day will be one month long, well, it depends on how you define month. But this would get too confusing.

I am mostly interested in other planets. I particularly enjoy learning about the moons of Jupiter. I have a cable connection so I am able to get pictures quickly and with ease.

Thankyou for your explanation of nutation and procession (By the way, I am 15 years old, so excuse my ignorance). I would like to learn more about the axis on which the axis of the Earth spins slowly around. Where was the word 'nutation' derived? I am having pizza for dinner tonight.

Do you believe there will be a huge catastrophic earthquake on the San Andreas Fault?

By the way, what time zone are you in?
Is there an atom clock on the internet?
What drew you into astronomy (was it an early interest?)?
I have this phobia about tidal waves. Nothing seems to scare me more than the thought of a huge expanse of water sweeping down on me. Do you have a phobia? Thank you for your reply.

Surely the length of the day should be measured in a nice figure understandable to those with no concept of hours. Like perhaps the distance light would travel in that amount of time.

Actually Spring does not always start on March 21st. In fact throughout the 21st Century Europe will see that happen only twice, and in North America it will arrive on the 20th every year (the 19th in some areas in 2004). For more on this see http://www.space.com/spacewatch/confounding_questions_021025-8.html

Summer is my favourite season. In fact, I am born on the longest day of the year in the southern hemsiphere (22 December). It's winter here in Australia but it must be around 20 degrees Celsius.

Is it the 22nd every year? Not sometimes the 21st?

I'm pretty sure it's the 22nd every year. Although my starsign does change annually...

In 1984: Sun reaches the most southern point of the ecliptic on Dec 21, 17:23CET. I think this also implies that it was the longest day, at least for europe.

I think the reason for this shift is the error that accumulates every year, and is compensated by the leap day.

I was born in 1987 so I was not aware of that fact. Thank you for that.

I'm not that much into astrology anyways...

I think that someone should tell spring that it begins on the 21st of March. It doesn't seem to understand the concept at all.

Dave

For meteorologists it begins on March 1st, by the way.