This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Path of the Sun in our Sky

by Dr Jamie Love Creative Commons Licence 1997 - 2018

As with most observational astronomy it is more convenient to imagine that the Earth is at the center with the Sun revolving around it - even though the opposite is true. It is convenient to imagine the Sun orbiting the Earth along a plane (the ecliptic). The Sun moves slowly along the ecliptic, changing its position, throughout the year. This causes the height of the Sun above the horizon, as well as its points of sunrise and sunset, and indeed its entire path through the sky, to change from day to day. However, one year later the Sun will have returned to the exact same position.
[That's not completely true. It takes 365 and a quarter days for the Earth to complete an orbit of the Sun, so the position will be slightly off depending upon the Leap Year. And you could extend the argument even further by dragging "precession" into this, but its effects are very small and we will talk about that feature in November.]

These changes can be best understood by starting with a simple example.
Imagine you lived at the Earth's equator and you plotted the position of the Sun at midday each day of the year. "Midday" (also called "local noon") is a rather amateur way of putting it, so let's be more precise. As the Sun moves from east to west over the course of the day it passes through your meridian. You will recall that the meridian is an imaginary line that passes between the two poles and directly overhead through the zenith of the observer.

This drawing illustrates you standing on the equator.
Your meridian goes from your northern horizon (at the top), directly overhead through your zenith, to your southern horizon (at the bottom).
Understand that this image is a cross section - the globe of the Earth curves through the screen. You can imagine sunrise occurs behind the screen, the Sun works its way towards the front of the screen, and the Sun will be within the plane of the screen as it passes through the meridian. That point, along the meridian, will be its highest point in the sky (that day).

In this next section I will remind you of the names of each Equinox as seen from the perspective of a Northerner by putting that part in parentheses. It might make for some awkward reading, so try it first by skipping the parentheses, and then reread it again to get the entire idea. Also I will be referring to only one date as the day of each event but you should understand that these Solstices and Equinoxes can occur on either the 22nd or 23rd of that month.

On December 22nd we will experience the (Winter) Solstice (for people in the Northern Hemisphere) and the Sun will be at its most southern position in the sky.

It will be 23.5 degrees south of the Celestial Equator as it passes through the meridian and it will not go any farther south. The next day, December 23rd, the Sun will be slightly farther north as it crosses your meridian. Each day it will cross the meridian a wee bit north of its crossing the previous day.

On the 22nd of March - the (Vernal) Equinox (for people in the Northern Hemisphere) - the Sun will cross the equator's meridian exactly overhead. That is, during the Equinox the Sun's highest point will be at the equator's zenith. Each day the Sun's meridian crossing will occur a little bit north of the previous day and on June 21st it will reach its most northern point. That's the other (Summer) Solstice (for people in the Northern Hemisphere) and the Sun will never go any farther north of that point in the sky. The next day the Sun will cross the meridian a wee bit south of that point and it will continue moving south each day.

On September 23rd - the (Autumnal) Equinox (for people in the Northern Hemisphere) - the Sun will cross the Celestial Equator as it passes through the meridian and it will continue moving farther south each day until December 22nd - the (Winter) Solstice (for people in the Northern Hemisphere).

Do these events affect the sunrise?

Yes!

Sunrise and sunset are also affected by the Sun's apparent path throughout the year.
On the morning of the (Summer) Solstice the Sun will rise at a point on the eastern horizon farther north than on any other day of the year. It will progress across the sky, though the meridian and set on the western horizon. Along the way it will travel through its northern most path through the sky. The next day it will rise slightly south of its sunrise point the previous day and travel across the sky slightly south of the path it took the previous day. Indeed, the Sun's path through the sky will be a reflection of its positioning throughout the year as described above for the meridian.

Let's see what that means to someone living in the Northern Hemisphere. Notice we are now switching our position to be NORTH of the equator. Also, note that we are changing our orientation so we can "see" the path of the Sun throughout the day. Specifically we will be looking at a southern horizon in the next few drawings.

On December 22nd the Sun will be as far south of the equator as it can possibly be.

On that day it will rise in the east at its southernmost point for the year and it will progress across the sky and set in the west, all along its most southern path. This means the Sun will move across the sky that day in a short arc.

The path the Sun travels on December 22nd, this arc, is the shortest path it travels through the sky.

However, the Sun moves through the sky at the same speed all year, all the time. That's because the Sun's "speed" through the sky is actually caused by the Earth's rotation and the Earth's rotation rate does not change. (Right? ) That means the day of the Winter Solstice will be the shortest day of the year because on that day the Sun travels less "distance" to get from sunsise to sunset. We (in the Northern Hemisphere) will have less daylight that day than during any other day of the year because the time between sunrise and sunset is shorter than for any other time of year.

Now contrast that with the path of the Sun during the Summer Solstice. On June 21st the Sun will rise in the east at a point on the horizon farther north than at any time of the year. The Sun will cross the sky and pass through the meridian at its most northerly angle. On that day, at "local noon", the Sun will be at it greatest height above the horizon. It will never appear farther north.

After passing the meridian the Sun will continue its long arc towards the west and eventually set on the western horizon at the most northern point possible. On the day of the Summer Solstice the path of the Sun through the sky will be longer than the path it takes any other day of the year. Therefore, the daylight during Summer Solstice will last longer than any other day of the year because the time between sunrise and sunset will be longer than any other day of the year.

Between the two extremes of Solstice we have intermediate day lengths.
During the two Equinoxes we have 12 hours between sunrise and sunset so we have equal amounts of daytime and nighttime.

The ever changing path of the Sun causes our seasons.

Hey, wait a minute. Why isn't the hottest day of the year on June 21st and the coldest on December 22nd?

That's because the Earth takes a long time to change temperature.
On December 22nd the Northern Hemisphere receives less daylight than during any other day of the year but it still has a lot of heat built up from the summer months, so the coldest day of the year doesn't occur until sometime after the Winter Solstice. Similarly, on June 21st there are still some parts of the Northern Hemisphere soaking up the Sun's rays and they haven't yet reached their maximum temperatures.
The great mass of the Earth, especially its atmosphere and bodies of water (oceans), causes our seasons to run a bit behind the Solstices. Nonetheless, it is these extremes of sunlight, caused by the Earth's tilt, that cause our seasons.

We can put numbers to all these events so let's look at some examples. "Meridian elevations" are a great way to understand the Sun's motion. (And, as you will see, this is a good exercise for understanding the seasonal change of any celestial object.)

On December 22nd - Winter Solstice in the North - the Sun will be 23.5o south of the equator as it passes your meridian. [Remember, the meridian is your "high noon" line.] If you were at the equator you would measure the Sun to be 23.5o south of overhead as it crossed your meridian. That means, IF you were on the equator (latitude 0o), the Sun's highest point for that day (which occurs at "midday" ) would be 66.5o above the southern horizon..
[That's 90o (the zenith) - 23.5o (the angle of the Sun from overhead due to the Solstice) = 66.5o (the angle above the southern horizon.]

If you are not at the equator you must take into account your latitude.
Imagine you are in New York City which has a latitude of 41o north.
That means the Sun will be much farther south, a full 41o farther south, than as seen from the equator. Therefore, the noonday Sun during Winter Solstice passes over New York City at an elevation of only 25.5o above the southern horizon.
[That's 90o (the zenith) - 23.5o (the angle of the Sun from overhead at the equator due to the Solstice) - 41o (how far north you are from the equator) = 25.5o.]
On December 22nd, the shortest day of the year, the buildings of New York City will cast their longest noon shadows.

Now consider the Vernal Equinox. At noon on that day the Sun will be directly over head, 90o, IF you live on the equator. But if you live in New York City, your northerly latitude will make the Sun appear to be much further south. Specifically, the noonday Sun will be 41o south of overhead or 49o above the southern horizon.
[That's 90o (the zenith) - 0o (the angle of the Sun from overhead at the equator during the Equinox) - 41o (how far north you are from the equator) = 49o.]

Lastly, think about the Sun's path on June 21st, the Summer Solstice. At the equator the Sun will be 23.5o north of over head as it crosses the meridian. At the same time New York City's northerly latitude will make the Sun appear to be further south than that. But how far south? We use the same logic as we did before but don't forget that the Sun is now north of the equator so we have to add its position to our zenith (not subtract it) and then subtract our latitude. So at local noon in New York City the Sun will be 72.5o above the southern horizon.
[That's 90o (the zenith) + 23.5o (the angle of the Sun from overhead at the equator during the Summer Solstice is added in) - 41o (how far north you are from the equator) = 72.5o.]
On June 21st, the longest day of the year, the buildings of New York will cast their shortest noon shadows.

These calculations are meant to give you a feel for how astronomers calculate the path of the Sun through the sky taking into account the movement of the Sun throughout the year and our location. By adding another layer of more complex math, the time of day and your longitude, you could calculate the EXACT position of the Sun throughout the day and throughout the year. Putting that same knowledge in reverse would give you a way to tell the time and date throughout the year using the position of the Sun.

Why have we spent so much time on this?
It's just the Sun. That has very little to do with astronomy.

WRONG!
I've been using the Sun because it's an obvious feature in our sky and you don't have to wait until nightfall to see these effects. But let's keep this in perspective. Remember, it's the axial inclination of the Earth that causes these changes. It really has nothing to do with the Sun. The Sun is just a star - and that's the important part!
You see, the Sun and all the other stars can be imagined as being part of a Celestial Sphere surrounding the Earth (even though that is not an accurate description). We can imagine that this Celestial Sphere, not the Earth, is the one with the tilt (even though that is not an accurate description). Like the Sun the Celestial Sphere shifts its tilt throughout the year (even though that is not an accurate description) - the Sun is just one more star on the sphere (even though that ... getting sick of this yet? )

All the stars will behave exactly like the Sun. They too will appear to change their altitude positions, such as their "meridian elevations", throughout the year. Unlike the Sun, distant stars do not affect our seasons, do not cast shadows and do not come out in the daylight. So it is much easier to explain these "tilt effects" by using the Sun. The calculations (above) may seem a wee bit excessive but astronomers do these calculations all the time, or have their computers do them, in order to figure out the altitude of a star.

This seasonal movement of the celestial sphere also affects how well we can see the planets
Recall that most of the planets orbit the Sun pretty close to the ecliptic because that is where the accretion disk formed billions of years ago. When the Sun is at its highest point in the sky, so too is the ecliptic and when the Sun is at its lowest point, so too in the ecliptic. This affects how well we can view the planets. Planet viewing is best when the ecliptic is highest in the sky because then the planets will be furthest above the horizon as they cross your meridian.

Now think about this. The Sun is at its highest point during the Summer Solstice - in June for folks in the Northern Hemisphere. However, daytime is the wrong time to try to see planets. (Right? ) The best time to view planets is when the ecliptic is at its highest point, but AT MIDNIGHT! Therefore, if you live in the Northern Hemisphere, the best time to watch planets is around midnight of the WINTER SOLSTICE! That means the best view of a planet occurs when it is at opposition (along your midnight meridian) on December 22nd (if you live in the Northern Hemisphere). That's when you want to look for Mars, Saturn and Jupiter because they will be well placed. Of course, it is very unlikely that there will actually BE a planet there at that time, but the ecliptic will certainly be well positioned, high in the sky, at that time. The "trick" is to understand how the seasonal variations will affect your chances of seeing a nice sight. On the other hand, June is an awful time to look for planets if you live in the Northern Hemisphere because the planets will be very low on the horizon AT NIGHT, along with the ecliptic. (Imagine, in your mind's eye, that a high altitude ecliptic at noon makes for a low altitude ecliptic at midnight. Or, skip back to our previous lesson and take a good look at the images with an eye to what we have been talking about here.)

But that's not all! During the Autumnal Equinox people in the north will find that the ecliptic is very well placed for viewing objects that are in the predawn sky, while the best time to see objects just after sunset is during the Vernal Equinox. If you live in the Northern hemisphere, try to watch Mercury and Venus during September before sunrise or in March after sunset.
And, of course, it's all reversed if you live in the Southern Hemisphere!
Yes, this is complicated, but read through it again to understand it and why it is so. Here's a summary chart to help you think it through.

Hemisphere
(latitude)

Northern

Southern

Catch ...

Sunset
(Vernal Equinox)

March

September

postdusk planets

Midnight
(Winter Solstice)

December

June

opposition planets

Sunrise
(Autumnal Equinox)

September

March

predawn planets

Noon
(Summer Solstice)

June

December

some rays, dude!

Remember, all these complications can be calculated exactly with some math. However, you don't have to do those calculations to appreciate the Sun's annual movement. You can measure it yourself and watch how the Sun's position changes from day to day or week to week.

Pick a spot in your neighborhood from which you can observe the sunrise or sunset. (Maybe a window in your home will give you a good view.) Over the course of a few weeks you will see that the Sun rises and sets in slightly different places.

If you have a compass you can follow this motion and plot the changes. You may be surprised to discover that the position of sunrise (or sunset) doesn't change equally from day to day.
NOTE: NEVER STARE AT THE SUN! Even the sunset and sunrise can be harmful if the Sun's disk is visible. (Position #9 in the image above would be very dangerous.) Just glance at the Sun and note its position.

You can also follow the Sun's motion throughout the day by following shadows. [Watching moving shadows is safe. Watching the moving Sun is dangerous!] Pick a nice sunny day one weekend when you have some time to kill. Place a pole into the ground far away from any buildings or other obstructions or find a convenient flagpole. Every hour place a rock at the tip of the shadow made by the pole. Tape to each rock a piece of paper with the time on it. This allows you to create your own sundial. If you have the time, you can make a sundial that spans from sunrise to sunset using the shadows cast by the pole. If you can leave the rocks in place for a long time, you will be able to see how the sundial drifts slightly "off" as the months go by. The point of the shadow will also shorten or lengthen in relation to the seasonal changes (Solstices and Equinoxes).

Seems like a lot of work!

It is, but it's also a lot of fun and a learning experience. Remember, the Sun is only one object on the celestial sphere. I like to use it in our lessons because it's so easy to observe the Sun and notice its seasonal variations in position. As you make these solar observations remind yourself that the Sun is always on the ecliptic (by definition). It slides along the ecliptic each day but is always somewhere along it. The planets will be somewhere near the ecliptic.

Oh, one other thing.
If you live at a latitude of 66.5o or higher the Sun's seasonal movements become so excessive that on the longest day of the year, the Summer Solstice, the Sun never sets. It just slides along the horizon as it moves towards the west where it should set, but it never hits that point. Instead it skirts along the northern horizon until it "rises" again the next day. On the shortest day of the year, Winter Solstice, the Sun never really rises, it just glows from below the horizon at the point where you might expect it to rise. That strange behavior is caused by the effects of a high latitude on the Sun's seasonal movements. If you look at a globe of the Earth you will usually find a special latitude at 66.5o north labeled "Arctic Circle". North of the Arctic Circle the Sun never sets during the Summer Solstice and it never rises during Winter Solstice. The farther north of the Arctic Circle you travel the more days you will have of either 24 hour daytimes or nighttimes.
You could work out the exact properties using what you have learned here but I won't go any further into it.
Of course, all these seasonal properties of the Sun are reversed in the Southern Hemisphere. The "Antarctic Circle" is at a latitude of 66.5 south.

By the way, the Moon has an axial inclination of only 1.6o. That means there are some craters near the poles of the Moon into which the Sun never shines.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.