I just received an Astronomy calendar that marks certain events of the year and it says that Earth will reach perihelion on the 4th of January... shouldn't that be Winter solstice? Is the true shortest day of the year going to be on the fourth?
Have a question for you astronomers!!!
- Thread starter Mucman
- Start date
um, I think the winter solstice was last Thursday. I don't know what perhelion is, but it sounds cool.
*PERHELION*
yep, that sounds cool.
*PERHELION*
yep, that sounds cool.
perihelion is when Earth is closest to the sun. Or when it is at one end of its elliptical orbit.
Assuming the solar sytem to be stationary, as Earth revolves around the sun it traces a path whose shape is elliptical.
The sun is not at the center of the ellipse, so one side of the ellipse lies closer to the sun than the opposite side.
Perihelion is the point of Earth's orbital ellipse that lies closest to the sun. Earth currently passes through that perihelion point each year on ~January 4.
Earth passes through the aphelion point, the point on the orbit farthest from the sun, each ~July 3.
As Earth revolves around the sun it does so tilted 23.5 degrees from the vertical -- Earth's axis is not perpendicular to it's solar orbit.
Currently, Earth's axis continually points in the general direction of Polaris, Alpha Canis Minoris, our so-called North Star.
On one day of the year, currently around Dec. 23, the component of Earth's axis in the northern hemisphere leans away from the sun at the full 23.5 degree angle, while the component of the axis in the southern hemisphere leans 23.5 degrees in the direction of the sun. This day is a Solstice.
Six months later, around June 22, the northern hemisphere is leaning 23.5 degrees in the direction of the sun and the southern hemisphere is leaning away. Another Solstice.
It is a coincidence that perihelion is so close to the solstice itself coincidently in December. It is a coincidence that has been repeated at fairly regular intervals over the age of the solar system.
The interval varies in length due to gradual, cyclic changes to the directions in space Earth's axis points toward above the northern and southern hemispheres (in part due to frictional sloshing of the earth's oceans and large lakes in their respective basins, etc.), and to gradual, cyclic changes to the elliptical shape of Earth's solar orbit due to gravitational interactions between Earth, the moon, and the sun, and much lesser with Jupiter and Venus.
The sun is not at the center of the ellipse, so one side of the ellipse lies closer to the sun than the opposite side.
Perihelion is the point of Earth's orbital ellipse that lies closest to the sun. Earth currently passes through that perihelion point each year on ~January 4.
Earth passes through the aphelion point, the point on the orbit farthest from the sun, each ~July 3.
As Earth revolves around the sun it does so tilted 23.5 degrees from the vertical -- Earth's axis is not perpendicular to it's solar orbit.
Currently, Earth's axis continually points in the general direction of Polaris, Alpha Canis Minoris, our so-called North Star.
On one day of the year, currently around Dec. 23, the component of Earth's axis in the northern hemisphere leans away from the sun at the full 23.5 degree angle, while the component of the axis in the southern hemisphere leans 23.5 degrees in the direction of the sun. This day is a Solstice.
Six months later, around June 22, the northern hemisphere is leaning 23.5 degrees in the direction of the sun and the southern hemisphere is leaning away. Another Solstice.
It is a coincidence that perihelion is so close to the solstice itself coincidently in December. It is a coincidence that has been repeated at fairly regular intervals over the age of the solar system.
The interval varies in length due to gradual, cyclic changes to the directions in space Earth's axis points toward above the northern and southern hemispheres (in part due to frictional sloshing of the earth's oceans and large lakes in their respective basins, etc.), and to gradual, cyclic changes to the elliptical shape of Earth's solar orbit due to gravitational interactions between Earth, the moon, and the sun, and much lesser with Jupiter and Venus.
Thanks for the explanation UG!
It just seemed commonsensical (to me) that the Earth would have its shortest day at perihelion... I guess I would need a diagram to show why the shortest day isn't on the perihelion.
It just seemed commonsensical (to me) that the Earth would have its shortest day at perihelion... I guess I would need a diagram to show why the shortest day isn't on the perihelion.
The explanation of what determines the shortest day of the year involves Earth's rotation, the variable velocity at which Earth moves along its orbit around the sun because of changing gravitational acceleration due to the elliptical shape of Earth's orbit, Earth's axial tilt, whether we measure the length of the day with a sundial or a clock.
In lieu of diagrams:
Darkest days of Winter.
Comparative lengths of longest/shortest days/nights.
In lieu of diagrams:
Darkest days of Winter.
Comparative lengths of longest/shortest days/nights.
What UG said...
But also, you have to remember that every day is of equal length...it is the daylight hours that change.
And the length of the day has nothing to do with how close or far away the earth is from the sun...at least in the miniscule amount our orbital distance changes.
For instance, if you were on Pluto, you would consider it a planet in perpetual darkness...
But also, you have to remember that every day is of equal length...it is the daylight hours that change.
And the length of the day has nothing to do with how close or far away the earth is from the sun...at least in the miniscule amount our orbital distance changes.
For instance, if you were on Pluto, you would consider it a planet in perpetual darkness...
Since we're on the subject of length(s) of the day, the second link -- Comparative lengths of... -- demonstrates that because of the optical characteristics of Earth's atmosphere the horizon is inadequate as a reference against which to measure the positions and motions of celestial objects.
Instead, astronomers use as a reference an imaginary line that connects the North and South cardinal points of the compass with the point at the top of the sky (the Zenith), a line they call the Meridian.
The Meridian is unique to each observer, but being a projection onto the surface of the sky, the Merdian behaves the same for every observer.
The Meridian rotates with the Earth. The sun, moon, planets and stars all sweep across the merdian, from east to west, as Earth turns west to east.
The interval of time required for a star to cross the Meridian, pass from sight behind the western horizon, reappear from behind the eastern horizon, and then return to the Merdian is the period of Earth's true rotation, 23h 56m 4.09s, and is called the Sidereal day.
Were it not for tidal interactions between the moon and Earth, and Earth's large bodies of water sloshing about in their respective basins, the rotation period of Earth -- the Sideral day -- would remain unchanged.
The sun also seems to move across the Meridian each day, at noon, to return to it the following day, at noon. This period is the Solar day. We average it as being 24 hours in length.
The solar day is not the same length as the sidereal day. It is longer. This is because as the Earth rotates it also is moving around the sun (it takes Earth 365 days to travel along its 360 degree orbit around the sun, moving almost one degree per day. Seen from Earth, the sun shifts eastward against the background of stars approximately one degree per day. The solar day is the sideral day plus 3m 56s, the time it takes the Earth to rotate that one additional degree (two apparent solar diameters). After a year, the sun is seen to return to the same place amongst the background of stars).
But because Earth's orbit is an ellipse, the rate at which Earth moves along its orbit is not constant. Near January, when Earth is closest to the sun and is moving faster, the solar day is longer than in July when Earth is farthest from the sun and moving most slowly.
There are a couple other day intervals of slightly different definition, but this shall suffice to make the point that day is a term not unlike megabyte.
Instead, astronomers use as a reference an imaginary line that connects the North and South cardinal points of the compass with the point at the top of the sky (the Zenith), a line they call the Meridian.
The Meridian is unique to each observer, but being a projection onto the surface of the sky, the Merdian behaves the same for every observer.
The Meridian rotates with the Earth. The sun, moon, planets and stars all sweep across the merdian, from east to west, as Earth turns west to east.
The interval of time required for a star to cross the Meridian, pass from sight behind the western horizon, reappear from behind the eastern horizon, and then return to the Merdian is the period of Earth's true rotation, 23h 56m 4.09s, and is called the Sidereal day.
Were it not for tidal interactions between the moon and Earth, and Earth's large bodies of water sloshing about in their respective basins, the rotation period of Earth -- the Sideral day -- would remain unchanged.
The sun also seems to move across the Meridian each day, at noon, to return to it the following day, at noon. This period is the Solar day. We average it as being 24 hours in length.
The solar day is not the same length as the sidereal day. It is longer. This is because as the Earth rotates it also is moving around the sun (it takes Earth 365 days to travel along its 360 degree orbit around the sun, moving almost one degree per day. Seen from Earth, the sun shifts eastward against the background of stars approximately one degree per day. The solar day is the sideral day plus 3m 56s, the time it takes the Earth to rotate that one additional degree (two apparent solar diameters). After a year, the sun is seen to return to the same place amongst the background of stars).
But because Earth's orbit is an ellipse, the rate at which Earth moves along its orbit is not constant. Near January, when Earth is closest to the sun and is moving faster, the solar day is longer than in July when Earth is farthest from the sun and moving most slowly.
There are a couple other day intervals of slightly different definition, but this shall suffice to make the point that day is a term not unlike megabyte.
I'm Typing, I understand that... it's just when Earth is at perihelion Earth's axis is tilted in a way that leaves the Northern Hemisphere with a short sunlit day (or so I thought was the case until I read UG's description).
As for sidereal and solar days... Sidereal days are longer than solar days? And is the sidereal orbit of the moon longer than its real orbit? I think I have heard that the sidereal orbit is something like 2 days longer...
As for sidereal and solar days... Sidereal days are longer than solar days? And is the sidereal orbit of the moon longer than its real orbit? I think I have heard that the sidereal orbit is something like 2 days longer...
<...or so I thought was the case...>
You thought correctly. Reread <...UG's description...>
<...Sidereal days are longer than solar days?..>
Nope. You're being dyslexic.
You thought correctly. Reread <...UG's description...>
<...Sidereal days are longer than solar days?..>
Nope. You're being dyslexic.
<...the sidereal orbit of the moon longer than its real orbit?..>
Damn. That's an interesting question I've not entertained before. The answer is no, it's sidereal orbit is shorter than its orbit around Earth, because Earth is moving around the sun at the very same time and the moon will meet up with the same star before it completes its 360-degree circuit of Earth.
Damn. That's an interesting question I've not entertained before. The answer is no, it's sidereal orbit is shorter than its orbit around Earth, because Earth is moving around the sun at the very same time and the moon will meet up with the same star before it completes its 360-degree circuit of Earth.
ok, my brain is hurting now
I need to really read over this stuff instead of glancing over it...
I need to really read over this stuff instead of glancing over it...
- Oct 28, 1999
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My tidbit of information, I think it was touched upon earlier - the planets don't travel around the sun at a uniform speed. As they reach the outer edges of the eliptical orbits they speed up. As they start to head into the long stretch of the elipse they slow back down.
<... As they reach the outer edges of the eliptical orbits they speed up...>
After they reach the outermost point of their orbits they speed up as they head back toward the sun.
<...As they start to head into the long stretch of the elipse they slow back down...>
On the way out, towards the outermost portions of their orbits, they slow down. On the other half of their orbits, heading towards the sun, they speed up.
I wonder why so many know computers, but so few know M-Theory?
Computers are so easy, of course.
After they reach the outermost point of their orbits they speed up as they head back toward the sun.
<...As they start to head into the long stretch of the elipse they slow back down...>
On the way out, towards the outermost portions of their orbits, they slow down. On the other half of their orbits, heading towards the sun, they speed up.
I wonder why so many know computers, but so few know M-Theory?
Computers are so easy, of course.
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