Can Someone Please Explain This To Me (Sunrise/Sunset Direction)

Factual Questions
1

Everyone knows the old line – the sun rises in the East and sets in the West. And, most of us know that that’s not exactly true - on most spots on the globe, the position on the horizon where the sun sets shifts a bit north or south (depending on the season) from day to day, reaching one extreme at the summer solstice and the other extreme on the winter solstice.

My understanding is that the sun can only be directly over a certain point on earth if that point is between the Tropics. The sun drifts northward (relative to Earth, of course) a bit each day until it reaches the Tropic of Cancer (23 degrees north and change) at the summer solstice and then begins drifting southward, crossing the equator at the equinox and eventually reaching the Tropic of Capricorn (23 degrees south and change) at winter soltice, whereupon it drifts northward again.

I live in NYC, which is at about 41 degrees north. Thus, for me, the sun should always be in the southern portion of the sky, since I live further north than the sun’s northernmost position relative to the earth. By my understanding and logic (which, we will soon see is flawed), the sun should rise somewhere in the southeast and set somewhere in the southwest.

However, that does not seem to be the case. I am an amateur landscape photographer and it is important to not only know when sunrise/sunset happen, but in what direction/where in the sky the sun will be positioned at that time. There is a wonderful tool that can be used for this, called Suncalc. However, I find that when I put in my location (or really just about any location on earth outside the Arctic/Antarctic circles), I see that the sunrise position is in the northeast and it sets in the northwest (for a day like today, in the summer. In the winter, it does shift to the southeast/southwest).

How is this possible? Shouldn’t it always be in the south for someone north of the Tropic of Cancer and always be in the north for someone south of the Tropic of Capricorn? How can sunrise be in the northeast for both myself and someone in a location like Patagonia (the southern tip of Argentina)? What simple astronomical fact am I missing?

Thanks,

Zev Steinhardt

(I considered the possibility that the software might be wrong, but another program that I use [The Photographer’s Ephemeris] confirms the results.)

2

The Sun will always be in the southern part of the sky for you when it is at its highest. But it can be in the northern half of the sky at other times.

At any point on the Earth (assuming a perfectly spherical Earth) on the equinoxes, sunrise and sunset are exactly due east and west. From March through September, sunrise is north of due east and sunset is north of due west, to a maximum in June. From September through March, both are south of East and West, to a maximum in December. Right on the Arctic Circle, at the summer solstice, sunrise and sunset are the same point, both due North. Likewise, on the Antarctic Circle, at summer solstice (i.e., in December), sunrise/set is at due South.

3

Zev, I’m 50 miles West of St. Louis and my app LunaSoCal shows this mornings sunrise azimuth as 58.9° which is definitely north of due east. My only explaination is that the earth isn’t flat. I’m sure someone with a better explanation will appear.

4

Miami Florida shows 63.5°

5 


                                           
      . ----- .                             
    .           .                           
  ..      X       .                         
 .    .            .                        
 |       .         Y                              
 |          .      |                              O
 .              .  .                        
  .               .                         
    .           .                           
      . _____ .                             

X=you at sunrise
Y=you at noon
O=sun

In the above summer diagram, consider the X point (sunrise). East for you is down-and-right on the diagram, parallel to the equator (dotted line). The sun is north of that direction.

Moving to the Y point (noon), the interesting direction for your question is now “up” rather than “east”. “Up” points radially outward to the sky. The sun is south of that direction.

6

I’ve noticed and wondered about this, too. In Chicago, the streets are scrupulously aligned with N/S and E/W. Yet I often see the sunrise or sunset occurring north of the E/W street I am walking along.

7

The reason is simple, but you have to visualize it a bit. We’ll work with a Terrocentric model (rather than heliocentric) because you’re here on Earth.

The sun is going around the Earth like a big hula hoop. But that hula hoop moves. As the seasons pass, the hula hoop moves North in the sky from Dec. to Jun, and South in the sky from Jun. to Dec.

But here’s the important part: the hula hoop doesn’t change its orientation. That means it’s sliding north and sliding south, not tilting north and tilting south. And unless you’re at the equator, the orientation of the hula hoop is not vertical to the horizon. In the northern hemisphere, the more north you go, the hula hoop stays tilted “down” a bit. That is, imagine you could reach up and grab the top of the hoop, and then tilt it to the south a bit, with the sides staying put.

So this means that, as the hula hoop moves north in the sky, both the top of the hoop, and the sides of the hoop move north. At the Arctic Circle, the hoop has tilted so far that the bottom of the hoop is briefly visible “behind” you as you face the top (the sun “sets” briefly directly to the North).

8

It’s easy to see how this works in the case of being at the Arctic circle on the summer solstice.

You’re pretty far north. And on that day, and in that place you see the sun rise due north at midnight, and see it set due north next midnight. At noon the sun will be in the southern half of the sky, but it will be in the northern half of the sky in the morning and evening.

9

Another way and perhaps the simplest way to think about it is that for parts of the summer centered on the summer solstice, north of the Arctic circle there is a period of time when the sun never sets at all, but just keeps going around in the sky and over 24 hours shines from every point on the compass – hence the “midnight sun”. Northerly latitudes south of the Arctic circle get this phenomenon only partially – the sun does set, but around the summer solstice days are long. The sun rises early from a northeast direction, travels a very long arc across the sky, and sets late in a northwest direction. The astronomical mechanics for why this happens have already been explained, but this might be a useful intuitive way of seeing it.

Of course the reverse happens in the winter. The sun rises from a southeast direction, remains relatively low in the southern sky, and traverses a relatively short arc to set early in the southwest.

10

The streets (at least what we call the Main Milers) here are also aligned on the N-S, E-W grid because the surveyors used the range and township lines of the PLSS. On March 20 and September 21 the sun sets right on the E-W streets. All summer it sets pretty well north and winter, pretty well south of right ahead. My brother lived west of work and complained how he’d be blinded driving home at 5:30. Me, I lived east of work so had the sun at my back on the way home.

The same thing happens in any city with streets more or less E-W. In Manhattan, it’s referred to as Manhattanhenge, but since the island’s grid is about 29-degrees off of true east-west, the alignment dates are May28 and July 13 instead of the Equinoxes. If you scroll down a bit in the M-henge article linked to, there is an illustration showing just how far north and south the sun sets on the Solstices.

11

One way I like to explain to my boy scouts about the sun and directions is that at noon (all times here are local, standard time, ignoring refraction, for the northern hemisphere), the sun will be directly to the south for those in the northern hemisphere between of the tropics and arctic circles. At midnight, the sun is directly opposite - if the earth was transparent, you’d see it at your feet (that’d be cool).

Halfway between noon and midnight is 6AM and 6PM, when the sun will be directly to the east and west respectively. The sun is always directly to the east at 6AM, which you can observe easily on the first day of the spring and autumn equinoxes, when the sun rises at 6AM.

However, the sun is always to the east, even if you can’t see it, such as during winter. If you could see through the earth again, the sun would be directly to the east at 6AM, but below the horizon. In the summer, you can see the sun at 6AM, and it’s directly east at that time.

On the equinox, the terminator - the line between night and day - extends directly from pole to pole, so the right-angle to that, where the sun rises and sets, is to the east at rising and west when setting. On this day, those on the equator see the sun rise at 6AM from the east, be directly overhead at noon, and set at 6PM directly to the west, giving 12 hours of sunlight. Others at different latitudes also get the same amount of sunlight at the equinox. On the first day of summer, the sun is over the tropic lines at noon.

For those of us in the northern hemisphere, during summer, due to the 23.5 degree tilt of the earth’s poles, the sun will start rising earlier, to the north of east. It has to be rising north of east, earlier and earlier until the first day of summer, which gives you more than 12 hours of sunlight in the summer. Likewise, to get fewer hours of sunlight in winter, the sun as to be rising to the south of east, some time after 6AM.

One good link I found about this is at Solar Path.

12

Heck. Buy a cheap globe of the Earth. The kind where the Earth is tilted. Put a ping pong ball on a stick, a few feet away, to represent the Sun. The Sun stands still, it doesn’t rise or set or do anything else. Make note of what the top of the axis of the globe points at in the room.

Turn the globe on it’s spindle axis. Notice which part of it’s surface points at the sun as you spin it. That’s sunrise and progress through the sky, and sunset.

Then pick the globe up, and move it to the exact opposite side of the Sun, and spin it again, without changing which direction the tilted axis of the globe points. If it was pointing towards the Sun before, it should be pointed away from the sun now. Note which points on the Earth point at the sun, and where it rises and sets.

if it helps, put a dot on New York, to make it easier to see.

You will very quickly be able to see directly, what the answers to your questions all are.

13

I found a surveying error on a local E-W thoroughfare during the spring equinox-the sun set a bit N of the highway as I drove W. Checked Google Maps, and yep they likely surveyed it based on magnetic north, not true north (the error here in NE Florida is smaller than most other places, but still significant).

14

Actually, “Chicago north” is 358.7 degrees, as you will notice on a service like Google Maps. This means the sunset is straight west on North Ave. or Irving Park, etc., on Sept. 26. (East-west streets in the North Loop, however, are aligned with the river, so have a “local north” of 359.8 degrees.)

Northern Illinois’s section lines were surveyed proceeding northeastward from Downstate Illinois. For reasons now lost to history, the line of reference, Third Principal Meridian, runs due north-south near Decatur, but a lot of error had crept in by the time they got this far north. This only applies to the portion of Chicagoland between the Indian Boundary Lines. North of Evanston or south of Dolton the section lines were based on other meridians and straighten up much closer to true north.

15

One way to think about this: On the north pole, in the summer, the sun circles uniformly above the horizon, so it sweeps the entire range of azimuth.

At the arctic or antarctic circles, at the appropriate solstice, the sun just dips to the horizon at true north/south…so it still circles the full range of azimuth. So at high latitudes, the sun pretty much rises and sets in the north in the summer.

As you travel southward, a sector of azimuth to the north is not swept by the sun. When you reach the equator, the sun will be 23.x degrees north or south of due east/west at sunrise/set at the solstices. (I so want the plural to be solsti!)

16

“Solsti” would be the plural of “solstus”.

17

Yes, for solstice, we’d have to go back to the fact it comes from the Latin second declension (neuter) noun solstitium, and the plural would be solstitia. So maybe Solstia. :stuck_out_tongue:

18

Actually the sun rises the furthest north and earliest right now, a couple of weeks before the solstice. It also sets the furthest north and latest right now. (6:11 AM and 8:28 PM). The opposite is true for the winter solstice.

19

Analemma.

20

Well done, Pasta! That worked for me, anyway.