Difference between autumnal and vernal equinoxes

[MrEbola]

TL;DR

Imaginary scenario. Let's say we get an astronomy expert to look at the sky with the naked eye on the 21 of September at 5pm in a geographical location known to him. And we ask him today is either the 21 September or the 21st of march. Would they be able to tell from the position and angle of the sun.

If no , what other information does the astronomy expert need to know to determine which equinox it is without access to any information about the location of stars or planets .

No more details to include

 

[Ibix]

You can see for yourself. Here's a night sky simulator:
https://www.timeanddate.com/astronomy/night/
You can set the date and time at the bottom of the picture of the sky. Does it make much difference if you set 21st March and change the year? Does it make much difference if you set September 21st and change the year? Does it make much difference if you switch between March and September?

 

[MrEbola]

The simulator shows a difference in the altitude and direction of the sun in the two equinoxes. If this is true why is it the case given that the tilt of earth in comparison to the sun is identical. Is altitude and direction parameters that can be assessed by the human eye on earth during the day time ?

 

[Ibix]

Because the Earth is on the other side of the Sun, and you are on the Sun-facing side of the Earth both times. At 5pm, then, someone on the equator is slightly above the orbital plane at one equinox and slightly below it at the other. Similar effects happen at other latitudes, but they're harder to describe in words.

 

[MrEbola]

Thank you so much for a very informative answer. So do you think you'd be able to tell in theory, let's say you've seen the same location numerous times at both equinoxes and you have a photographic memory.

 

[Baluncore]

The difference in altitude of the Sun is very small between the autumnal equinox and the vernal equinox.
There is, a small difference in the position of the Sun in time. That difference amounts to about 5° of azimuth or 20 minutes in solar time read from a sundial. Given a clock and a sundial, the astronomer would notice if the sundial was reading ahead or behind the clock by about 10 minutes.
https://en.wikipedia.org/wiki/Analemma#As_seen_from_Earth

 

[Vanadium 50]

Couldn;t you figure it out by trying to balance an egg?

Note to the humor impaired: you can balance an egg on its end on the vernal equinox. And the autumnal equinox. And Christmas. And Arbor Day. And Chester A. Arthur's birthday. And...

 

[Baluncore]

Let's say we get an astronomy expert to look at the sky with the naked eye on the 21 of September at 5pm in a geographical location known to him. And we ask him today is either the 21 September or the 21st of march. Would they be able to tell from the position and angle of the sun.

The Sun rotates on its axis, depending on solar latitude, every 25 days at the equator, through to 34.5 days at the poles. If a sunspot was visible and the astronomer could determine its path along the face of the Sun during the rotation, then the axis of solar rotation determined, would differ when viewed from Earth, by 2 * 23.5° = 47°, between the two equinoxes.

 

[Vanadium 50]

If a sunspot was visible and the astronomer could determine its path along the face of the Sun during the rotation,

Yes, nut this seems hard. If you can observe long enough to see the sun rotate, can't you tell if days are getting longer or shorter?

 

[Baluncore]

Yes, it is hard, but it is the biggest difference between the equinoxes, that does not involve the completely different background constellations of stars.

If only we could measure the 9×10⁻⁶ solar flattening due to rotation, but there is no sufficiently accurate theodolite to resolve the 0.03 arcsec. Any flattening would be buried deep in the Earth's atmospheric refraction.

 

[Vanadium 50]

It might be easier to measure the size of the sun's disk, which is a fraction smaller for the (northern hemisphere) autumnal equinox than the vernal.

 

[Baluncore]

It might be easier to measure the size of the sun's disk, which is a fraction smaller for the (northern hemisphere) autumnal equinox than the vernal.

How much variation is there in angular diameter between the equinoxes?

The diameter must be measured in azimuth, at the same elevation, since differential atmospheric refraction will be significant in the vertical.

 

[Vanadium 50]

How much variation is there in angular diameter between the equinoxes?

Fraction of a percent. The earth's orbital eccentricity is 1.7%, but perihelion is in early January. If perihelion were at the solstice, the effect would be zero.

 

[Baluncore]

Fraction of a percent.

Assuming 0.25% of 31 arcmin = 4.6 arcsec.
A theodolite can measure 1 arcsec, so it might just be possible.

But the Sun does not stand still while the two accurate measurements are being made and subtracted. To get an accurate measurement of the Sun's diameter, it would be necessary to split the solar image into two. The two images of the Sun are then separated, so that the opposite limbs of the Sun osculate. That instrument could be screw adjustable, calibrated to read out in arcsec, on either side of the approximate 31 arcmin diameter. Both images move together, so only one observation and adjustment is needed per measurement.

But what optical system would be needed for the human eye to judge the grazing incidence of the limbs, and how rough or distinct are the sides of the visible Sun.