The sun has finally begun to rise earlier. (Just as an aside, it actually does not begin to rise earlier until January 11, not at the winter solstice.) The weather reports say it rises at 7:17am now. Sets at 5:47 pm. But if I’m on top of a hill, I can see it rise earlier and set later. Just at what altitude are these sunrises and sunsets taken? I live at near sea level, so I guess sea level will do for me. But what about Chicago, or anywhere inland? Chicago, for example, has varying altitudes. What altitude is used for its sunrises and sunsets? Where the weather bureau is located?
That’s a good question. My WAG would be that it’s calculated at sea level, since that’s a good universal standard.
I don’t know if your question will be answered here, but you might want to poke around here.
http://aa.usno.navy.mil/AA/faq/docs/RST_defs.html#top
These are the people who REALLY have the time. The part on “Horizon” at the top should give you an idea of what is involved.
I get the impression that altitude isn’t factored in because the difference would be quite small.
Upon thinking about this more, I think that the altitude of the observer isn’t important. You’re still looking out on to a horizon for a sunrise. What would make a different is if there is a big mountain on the eastern horizon. That could delay sunrise for a particular observer. I believe that astronomers don’t take that into account because they can’t know what is going to be blocking the view of any one particular person.
I actually called <some organization - USGS?> when I was living in Hawaii and asked that same question; I was told they go under the assumption that the sun is rising at sea level.
It bugged me because sunrise on the Wainae coast (which is overshadowed by the Wainae mountain range was several hours later than sunrise on the Diamond Head side of Oahu.
As long as you’re on a level surface, the absolute elevation should make very little difference. It’s just changes in elevation you need to worry about, and those are ignored.
A couple of dimly remembered “facts” about sunrise:
- the moment of sunrise is defined with reference to the upper limb of the sun, not the centre.
- atmospheric refraction gives the sun an apparent lift (allowing you to see “over the horizon” as it were).
Taking these two facts together, at sunrise the elevation of the sun is actually about minus three quarters of a degree (depends on atmospheric conditions) and the sun is visible a few minutes earlier than the “elevation = 0” condition.
Mountains or other obstacles are not taken into account for sunrise calculations; there are hill-farms in this part of the world that have no sun for over a hundred days a year, but sunrise is officially the same for them as for their neighbours across the valley.
Isn’t it more an issue of longitude rather than altitude? It sounds like the relevant question should be, “where (east-west coordinates) along the time zone is the sunrise being calculated from?”
After all, using Chicago as our example, given that the whole city shares the same clock-time the sun rises at the easternmost edge of town earlier than the westernmost edge of town.
Right? Or am I missing somethin’ here?
That’s right. For every one degree further west, the sun rises (and sets) 4 minutes later. One degree is about 111 km at the equator, multiply by cos(latitude) for your location. So the sun rises a minute or two later in the western suburbs than the eastern suburbs.
The width of a degree of longitude at the equator is about 69 miles or 111 km (it shrinks down to zero at the poles). The width of a degree of latitude is 69 miles everywhere.
I’m at the latitude of about 39 deg N. So taking cos(39) (=0.777) times 69 miles makes about 53.6 miles per degree of longitude. The earth’s rotation takes four minutes to proceed through that distance here.
So dividing it by four means that every 13.4 miles translates into one minute of solar time on the latitude of Washington, DC. That distance east of here means that, from the Capitol, the sun rises one minute earlier in Bowie, Maryland and one minute later out in Vienna, Virginia. You can do the math for your own location.
My initial question appears to be answered, along with some other observations. The link states that it’s taken at sea level. Longitude is important, of course, but in the smaller cities not that much of a factor. Altitude does make a difference, but some have put it as changes in altitude. No city is completely flat. Sunrise or sunset on the ground will be later than if you were to go to the 3d floor. Weather bureaus have their instruments in a tower, which is well above the ground.
I mentioned that the sun actually does not rise any earlier in the morning at the solstice, but there is a delay of 2-3 weeks, altho it does begin to set later. Can any one explain that?
Longitude also plays a part at this time of year, as the terminator sweeps in from the north east. There is a great program, GeoClock, that you can get from http://www.geoclock.com that really shows this.
Here’s the answer to barb’s question, it’s too complicated to provide a meaningful excerpt.
East-west movement of the sun is due to the combined effect of the inclination of Earth’s orbit and the fact that we orbit the sun slightly faster near perihelion, and slightly slower at aphelion. Solar time thus runs slightly ahead of clock time (up to 4 minutes) from mid-april through mid-june and (up to 17 minutes) from september through january, and slightly behind clock time (up to 15 minutes) from january through mid-april and (up to 6 minutes) from mid-june until september.
This is explained quite well, both graphically (with QT animations) and mathematically, at www.analemma.com.