My son asked me this question the other day when the sun was setting, and I said “Umm… I get back to you on that” . Obviously, it’s generally the location in the west, but is it exactly the same spot every day given the earth’s precession wobbles etc.?
I asked something akin to this in a boneheaded sundial question a couple of weeks back with some complex answers, if you can find it.
The sun only sets precisely in the west at the equinoxes. In the summer (in the Northern Hemisphere) it sets north of west, in the winter south of west. It’s due to the inclination of the earth’s axis, nothing to do with precession.
No. Its position at rising and setting shifts with where its visual image is on the “equinoctual colure” – the sine-wave-shaped curve that represents its movement north and south over a year. (Remember that we’re talking the “visual sun” here – the image that rises and sets and moves north and south with the seasons – though of course the Earth’s rotation and tilt are what govern its behavior.)
I believe it moves north and south over about 45 degrees of arc, relative to the horizon, between the two solstices, although I’ll guess that this is actually an approximation that the actual value requires spherical trigonometry to define.
I think it’s 47 degrees - twice the inclination of the Earth’s axis. No complicated maths necessary.
It is safe to say the sun never sets in the same place two days in a row. After the winter solstice, the point of sunrise moves a little south each day, until the summer soltice, when it begins to move north again.
The direct answer is “no”, I’m pretty sure - and I don’t think where the sun hits the horizon is limited to only 47 degrees change. The 47 degrees would apply only to the “tilt” of the sun’s arc, not where it hits the horizon, I think. It’s actually a fairly complicated 3-D geometry problem.
Hm, this could have the makings of a home scientific experiment, where you and son find a way to mark the sun’s progress through the year…
But the kicker in my mind are the stories about life in the arctic circles, where some parts of the year the sun never sets, other times of the year it never rises, and does curious things during the transitional periods.
That’s its declination relative to the celestial equator. However, Polycarp was speaking of the Sun’s amplitude relative to due east or due west on the horizon.
You do need trigonometry to calculate the amplitude, which of course varies with latitude. (Or you can just observe it, Stonehenge-style!) At my latitude of 42 degrees, the Sun rises and sets 32 degrees north or south of due east and west on the solstices.
Nope, this would only be true at the equator. You can see this by noting that at the Arctic Circle, the Sun “sets” at due North on June 21, and “rises” at due South on Dec. 21.
The actual formula (using spherical trig., as Polycarp suggested) is that cos(a) = sin(d) / cos(l), where a is the angle from East/West at which the sun rises & sets on the solstices, d is the declination of the Sun on the solstices (23 degrees & change), and l is the latitude.
Assuming that Astro and his son are not the same height, would it make any difference which one took the reading? I don’t know much about this stuff, but I have read that a sailor taking a sighting at sea must adjust for how far the deck is above the water.
You see, I know barely enough to ask this question. I don’t know how to use an astrolabe or whatever you call the modern tool that replaced it. No, not the GPS, the one with the mirror and tiny marks on an arc.
I think you mean a sextant.
I did exactly this experiment in high school physics. Made a sketch looking out of my bedroom window, sat in the same place and indicated where the sun set each night for a while (a week or two IIRC). It did indeed set in different spots over the course of my observations.
In principle, yes. But in practice… Take a look at a sunset some time, and then crouch down to kid-height, and see if your view changes. Any difference will be far too small to detect with the naked eye.
The mariner’s astrolabe and the sextant were/are used to measure the altitude of objects above the horizon. This question concerns the position of the Sun at sunset, when altitude by definition is zero, so neither would be necessary. At sunset we’re concerned only with the azimuth of the Sun, which you can measure with a protractor provided you have a way to align it accurately with due north.
The height of the observer will make an ever-so-slight difference in azimuth at sunset, since the sunset itself will come ever-so-slightly later as you gain elevation above your surroundings. One may safely ignore this difference for any practical purpose.
Obviously, the answer that you’re getting is “No,” which is the correct one.
The best answer to your son, IMHO, is “Let’s find out.” That way, he learns, rather than just believes. Set up a way to track the point of sunset and keep a record over time. The winter solstice is coming up in a few months (just before Christmas), so he can observe the setting point reverse itself and start going the other way.
Don’t ask him to take someone’s word for it. Help him find out for himself.
THAT’S “fighting ignorance.”
Thank you. I couldn’t come up with the word “sextant.” All I could get was “sexton,” the guy in charge of a cemetary. The folks in a sexton’s care have no need for navigation, so I knew that wasn’t right.
What exactly was Stonehenge supposed to be measuring?
That’s pretty much what I was thinking. Simple observation from week to week will show your son that the setting point of the sun moves throughout the year. My nephew had to do a similar experiment for his school science class last year.
As others have mentioned, it doesn’t set in the same place every day.
A good tool to get an idea of how the sun’s path changes every day is a sun chart like the one you see. (An excellent source of finding these is The Passive Solar Energy Book by Edward Mazria. )
Unfortunately, it’s a little hard to read the one they have posted on this site, but you can get the general idea. A sun chart plots the altitude angle of the sun vs. the azimuth angle for each hour the sun is up.
“Azimuth” is a measure of how much of an angle the sun is away from due south. At solar noon (which will not be equal to clock noon), the sun will be at its peak height for the day, and at an angle of 0 degrees azimuth.
Look at the chart. It has seven different lines to show the sun’s path at 12 different months of the year. Note that each line (except the bottom one, which represents the winter solstice, and the top one, which represents the summer solstice) will represent two different dates — that’s because the sun’s path at, say, two weeks before the equinox is the same as the sun’s path two week after the equinox. The sun just travels back and forth between the path at the two equinoxes during the course of the year.
Note that the sun chart is individualized for a particular latitude. For instance, I live at 36 degrees north latitude; in summer, the sun will peak at about 78 degrees above the horizon at noon here. If you live farther north, it won’t get that high. At 56 degrees north latitude, it will max out at about 57 degrees or so.
Anyhow, again, if you look at the example sun chart for 44 degrees north latitude, and look at the lowest sun curve for Dec 22, you will see that the sun rises at about 55 degrees east of south, and travels up through the sky. It reaches its peak altitude at noon, at around 22 degrees. It will set at about 55 degrees west of south, as the sun’s path is symmetrical around south. Every day during the year between the winter solstice and the summer solstice, it will rise just a little bit further east of south than it did the day before. When it reaches the summer solstice, it rises (and sets) a little over 120 degrees off south. It reaches it maximum summer height of around 69 degrees at solar noon on the solstice.
If you see the semi-vertical dotted lines going through the different sun curves, those represent the time. The vertical line going through the center of the curve is solar noon; the dotted lines each represent one hour of time. Thus, you can see that on Dec 22, the sun rises at roughly 7:30 (solar time.) In the summer, sun rise is closer to 4:30 or so.