Sunrise, Sunset

[csandste]

Must have been asleep during ninth grade general science. I'm sure many of you can explain what I dozed through.

Looked up sunrise and sunset yesterday (Sunday) in St. Louis.

Sunrise was 6:49. Sunset was 6:56.

That's more than twelve hours of daylight. I thought days were shorter than nights fall and winter.

All I can think of was that the additional time represents the diameter of the sun. When a bit of it is above the horizon it's sunrise, when the last part goes down it's sunset.

Is that it, or is there something else I'm missing?

 

[CourierDriver]

Just enjoy the joy that you get to see it each day....Saw many a sunrise and sunset at sea while boating with the Navy...great views and the moon is unreal at sea also..

 

[ZeeOSix]

The Equinox technically occurred sometime mid day, so that's why the slight difference in sunrise & sunset times.

 

[Shannow]

Tomorrow here is 5:47 AM to 5:57 PM
Equinox was yesterday at 11:54 AM

 

[Shannow]

Originally Posted by csandste
All I can think of was that the additional time represents the diameter of the sun. When a bit of it is above the horizon it's sunrise, when the last part goes down it's sunset.

Is that it, or is there something else I'm missing?


missed your key point, sorry...

You are spot on.

https://www.timeanddate.com/astronomy/equinox-not-equal.html
Quote
On the equinoxes, the geometric center of the sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.
However, ‘sunrise' is defined as the moment the upper edge of the sun's disk becomes visible above the horizon - not when the center of the sun is visible. In the same sense, ‘sunset' refers to the moment the Sun's upper edge, not the center, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.

 

[skyactiv]

All I know is that there is less daylight each day after the first day of summer until the first day of winter. When winter starts, we gain more daylight each day until summer starts.

 

[Astro14]

Because of the angle of inclination of the Earth's axis, the length of a day varies with latitude (how far north, south) and time of year (where that axis tilts in relation to the orbit around the Sun). The orbital path is known as the ecliptic, and it's a defined plane. Much of astronomy is defined relative to the ecliptic.

The longest, or shortest day, is the solstice. Summer solstice is the longest (roughly 22 June) in the northern hemisphere. Winter solstice is the shortest, roughly 22 December. Longest and shortest also correspond with the apparent height of the sun at noon (again, that angle of inclination). Highest elevation on the summer solstice, lowest in winter.

The ancient peoples knew this. Monuments like Stonehenge, or Newgrange (Ireland, even earlier) are aligned to the sunrise on the day winter solstice (roughly, there has been some precession, or wobble, in the axis...which throws things off by just a bit).

The vernal and autumnal equinox are the days (March 20 and September 23, roughly) are those points in the Earth's orbit when the days are equal in length within the night, or the sun is halfway through its annual change in elevation.

The definition of the Arctic and Antarctic circles are based on theses solstices, too, by the way. On the solstice, at the precise latitude that defines the Arctic or Antarctic the sun will either stay above the horizon for the entire day, or not been seen at all. Again, winter, summer, north, south.

What's happening for us, in the Northern Hemisphere, is the opposite of what's happening in the Southern Hemisphere, so, as the days get longer in one, they're getting shorter in the other. Equinoxes are the same day in both.

 

[Astro14]

And yes, to your point, the sunrise, sunset numbers are predicated on the size of the Sun's disc, so they're not precisely equal, as there is "sunlight" for slightly longer because of that size, and when the disc breaks the horizon going up/down.

So, even though the inclination of the Earth's axis on the equinox is halfway through the annual change, and point neither towards the Sun, nor away from it, and the Sun's declination is halfway through its rise (or descent), there is sunlight for a few minutes more.

If we measured "sunrise" as the moment that the disc was halfway visible, and "sunset" as the moment that the disc was halfway visible, the times would be precisely equal on the equinox.

Now, 24 hour day, with 360 degrees of travel for astroical objects means that they move at roughly 15 degrees per hour. One degree every four minutes. Your fist, held at arm's length is about ten degrees..so, things in the sky will move about the distance of your fist every 40 minutes...anyway...

The Sun has an apparent size of about half a degree. So, the disc is visible about 1/4 degree before the center, or about a minute early. And it will set about 1/4 degree after the center, or about a minute late.

Every day, every time, there are some more subtle effects of geometry, but close enough, because of how we define sunrise and sunset. But the above math presumes that the sun rises at a 90 degree angle.

Now, in Saint Louis, at 40N, will have the Sun transiting the sky at an angle of about 45 degrees... I.e. the sunrise is happening at an angle of 45 degrees to the horizon.

That means that the disc is visible at 1/4 of a degree early, but on a path that makes that angular distance visible about a minute and a half early. Think of a 45 degree right triangle. Sitting right on the horizon. The sun is sliding up the hypotenuse. So, while it's going across the hypotenuse at 15 degrees per hour, as above, when looking at the vertical axis, it's moving up the vertical at about 10 degrees per hour (Sin 45 ~ 0.7). and that 1/4 degree early just became 1 1/2 minutes.

And there is all of the variation you're seeing.