How do astronomers define a solar day?

Answer 1

== == A solar day is the length of time between two successive passes of the sun across the same spot in the sky (e.g. crossing the meridian, overhead). That time period is, on average, 24:00:00, hours, or one mean solar day.

A sidereal day is the length of time between two successive passes of the fixed stars across the sky. That time period is 23:56:04, or one sidereal day.

The difference occurs because, over the course of a year, the earth does one revolution, or "lap" around the sun, so that there must be one less solar day (365.25) per year than there are sidereal days (366.25). Spread over the year, comes to a difference of about four minutes each day.

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Answer 2

The normal (solar) day, on Earth, is calculated by timing the rotation of the Earth relative to the Sun, which moves slightly eastwards across the sky each day due to the Earth's orbital motion.A A sidereal day is the same rotation, but this time relative to the stars, which are stationary,to all intents, as the Earth orbits the Sun.

The difference between the two "days"is only about 4 minutes on the Earth, but planets like Mercury and Venus have a much bigger difference, because they rotate slowly.

Answer 3

On Earth, the sidereal day is shorter, by about 4 minutes.

As the Earth spins on its axis, it also goes along in its orbit around the Sun. The Earth actually spins one time in about 23 hours, 56 minutes - but in that time, the Earth has moved along in its orbit so that it takes four additional minutes of spin for one point on Earth to be pointing straight at the Sun again.

But if you look at the stars, a particular star rises every 23 hours 56 minutes.

Things are different on Venus, because that planet has "retrograde rotation".

On Venus it is the solar day that's the shorter day.

Answer 4

The same way they define an Earth Day.

They calculate the time it takes the Sun to make one complete turn on its axis.

I think the question isabout the thing called a"solar day" on Earth, notabout the Sun's "day".

The solar day is simply our normal 24 hour day. It's based on the position of the Sun in the sky. That's why it's "solar".

It's about 4 minutes longer than the Earth's rotation period. It depends on the

Earth's orbital motion round the Sun as well as the rotation of the Earth.

It's the time between two successive appearances of the Sun on the observer's "meridian".

A meridian is an imaginary circle in the sky passing through the north pole of the sky, the observer's zenith (directly overhead), and south pole of the sky.

The solar day depends on both the rotation and the orbital movement of the Earth.

There are some natural variations in the length of the solar day caused by the Earth's movements. So, an average for a whole year is taken.

This gives us the average day length, called a "mean (average) solar day".

Our clocks use this "mean solar day".

Also, you can define a solar day in a similar way for other planets, besides Earth.

Answer 5

Sidereal day is a term used in Astronomy is describe the length of time that passes between a fixed star passing a line of longitude.