We usually think of the
Earth as having a North Pole pointing straight up (towards Polaris)
with its equator located exactly 90 degrees (90o) from it. The
Earth's latitudes and longitudes are based upon this system. We
use the equator to divide the Earth into Northern and Southern
Hemispheres.
You will recall (from lessons in February about declination) that by projecting the equator into the sky we get the Celestial Equator right above the Earth's Equator. Like its Earth-bound counterpart, the Celestial Equator divides the sky into Northern and Southern Celestial Hemispheres. The celestial coordinates of declination are based upon this system. | ![]() |
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You may recall that ORION straddles the celestial
equator.
His belt runs almost exactly along the equator.
Note that you can still see Rigel from almost anywhere on the Earth's Northern Hemisphere because it's much farther away than illustrated here, so the difference in angle is much less obvious. However if you were at the North Pole, or very close to it, you would be too far north to see Rigel. |
This system of celestial coordinates is perfectly adequate for
explaining and understanding most of astronomy but not all of
it.
To understand why and how the Sun shifts its apparent path through the sky as the months go by, you must learn about the Earth's tilt.
Think about the geometry of these two important planetary
values.
The rotational axis of the Earth is defined by how the
Earth rotates. This axis is constantly pointing its northern pole towards Polaris. The Earth's orbital plane is the (flat) plane it "draws"
in space as it orbits around the Sun.
This plane evolved
from the accretion disk as the planet formed billions of years
ago. All the planets formed from the same accretion disk and have
similar, although not exactly the same, orbital planes.
You might expect that the Earth's rotational axis would be perpendicular
to its orbital plane but you would be wrong!
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Why (not)?
No one knows for sure.
One ideal is that small differences in the distribution of mass
in the accretion disk and the accretion process caused tiny imbalances
in the protoplanets. These slight imbalances became more pronounced
as they grew into planets.
Another theory, which most astronomers accept, is that collisions with other masses (planets, asteroids, etc.)
tipped the rotational axis from its previously perpendicular orientation.
Regardless, the Earth is tipped 23 and a half degrees (23.5o)
from its orbital plane and that complication produces our seasons
and gives us several new astronomy definitions to learn.
Because of this tilt, as the Earth orbits the Sun, the amount of sunlight falling on each hemisphere changes and the daily path of the Sun through the sky also changes.
This diagram might help you understand. I have shaded the night side
of the Earth(s) in this image to help you to imagine
how the sunlight would fall on the Earth from each of these four
positions.
Look at the Earth's orientation on the left side of this picture.
Half a year later, on the right side of the diagram, the South Pole has daylight all day (all 24 hours) while the North Pole is in complete darkness. | ![]() |
It's the tilt of the Earth that causes this effect and our seasons.
Hmm, do you mean we have seasons because at those extremes one pole is closer to the Sun than the other?
No! While it is true that at these two points in the Earth's orbit we have the poles at their extreme orientation with respect to each other and the Sun, but it's the angle (not the distance) that is important. That's a common mistake so let's spend some time on it.
You can see that on the left side of this image the Earth's North Pole is closer to the Sun than the South Pole and vice-versa on the right side. But that difference is very, very small compared to other orbital properties, especially perihelion and aphelion.
You will recall from our lesson on orbits that the orbit of the Earth has an eccentricity of 0.017. The distance from the Earth to the Sun changes and the difference between its perihelion and aphelion is 0.033 Astronomical Units (AU). You will also recall that an AU equals 150 million kilometers. That works out to be about 2.5 million kilometers (150 million kilometers times 0.017) difference between aphelion and perihelion. The Earth is only 12,750 kilometers from pole to pole (not at all to scale on this drawing). So you can see that if distance from the Sun was important than we should have extremely cold weather at aphelion and very hot weather as perihelion because the perihelion/aphelion difference is about a thousand times greater than the difference between the two poles. | ![]() |
That doesn't happen. Indeed, you will recall that the Earth's aphelion occurs in July so, if distance was important, the entire planet should have winter in July.
It's the angle of the Sun in the sky and how long it stays in the sky (per daily rotation) that causes our seasons. On the left side of this image we see the Earth during its Northern Hemisphere summer. It's summer in the North because the Sun spends more time shining on the Northern Hemisphere and does so from an angle that is closer to perpendicular to the Earth's northern surface. It is this angle that helps the Earth to be warmed by sunlight.
This makes sense for two reasons, both of which you can experience first hand.
If you want to get warmed by the Sun you should stand in the sunlight for as long as you can.
Also, you should try to get the Sun's rays to strike your body perpendicularly on as much of your surface as possible.
Both of these events occur to the Northern Hemisphere on the left side of my drawing and to the Southern Hemisphere (a half year later) on the right side of my drawing. Meanwhile, the opposite properties (shorter day lengths and bad angle to the Sun) occur at the opposite hemisphere.
On those two days of the year (two parts of the Earth's orbit) the Sun appears to
have migrated to its most northern (on the left) or southern (on
the right) positions in the sky. During those extremes one hemisphere
gets maximum sunlight (because the "days" are longer)
while the other hemisphere gets minimum sunlight (because its
"nights" are longer). At the poles those extremes are
even more obvious with one pole getting daylight no matter what
time of day it is, while the opposite pole experiences "nighttime"
all day long.
As the Earth moves around its orbit, from one extreme to another, the orbit takes the Earth to positions (at the top and bottom of the drawing) where the sunlight is "shared" equally between the two hemispheres and everywhere on Earth has daylight that lasts 12 hours and nighttime that last 12 hours. (Admittedly, my poor drawing doesn't show this well but if you think of the plane in the proper orientation, this becomes clear.) | ![]() |
At these two points in the Earth's orbit the tilt is actually perpendicular to the Sun and the Sun will move across the celestial equator. At (local) noon the Sun will be directly overhead for someone on the equator. On the other hand, the Sun will appear to be either north or south of overhead for someone at a latitude that is not the equator's (0o).
Naturally, we have names for these important events.
Solstice is the day of the year (or the part of the orbit)
when there is maximum daylight at one hemisphere and
minimum daylight at the other. Solstice occurs twice a year - both extremes
to each hemisphere.
The day of the year (or part of the orbit) midway between the two
Solstices is called the Equinox and on that day all parts
of the Earth experience equal amounts of daylight and nighttime
(12 hours of each). There are two Equinoxes each year.
The time (point in the orbit) of maximum sunlight for a hemisphere is called the Summer Solstice. For folks
in the Northern Hemisphere it occurs around June 21st but for folks in the Southern Hemisphere the Summer Solstice occurs six months later around December 21st.
(I say "around" because the Earth's orbit is 365.25 days and the Leap Year causes the exact dates of Solstice and Equinox to jump around a day or so.) On the day of the Summer Solstice the Sun appears to be as close to the pole as possible, causing it to spend more time in the sky so folks in that hemisphere will enjoy the longest "day" of the year. On that same day, at the opposite hemisphere, the Winter Solstice occurs and folks in that hemisphere experience the shortest day of the year. As the earth moves from Summer Solstice to Winter Solstice the days (daylight) grow shorter and the Sun rises less high in the noon sky. (We'll come back to this in our next lesson.) | ![]() |
Eventually the daytime is equal to the nighttime and we experience the Autumnal (autumn or "fall") Equinox. As the Earth continues in its orbit the daylight gets shorter and shorter as the Sun sinks lower in the sky each noon until it reaches the Winter Solstice. After the Winter Solstice the day length starts to slowly increase. Three months later (or six months after the Autumnal Equinox) the Earth's tilt is again perpendicular to the Sun's rays and we have the Vernal (or "spring") Equinox.
As you can see, the seasonal names of the two Equinoxes and Solstices depend on
which hemisphere you are in at the time.
On the 22nd or 23rd of September (the exact day depends on Leap Years)
the Earth will be positioned so that the Sun's rays strike it
evenly and there will be equal amounts of daylight and nighttime everywhere - both north and south. People in
the Northern Hemisphere will be moving from summer to winter so
they have an Autumnal Equinox but folks living in the Southern
Hemisphere have everything reversed - they are moving from Winter
towards Summer, so they have the Spring Equinox. It's the same
Equinox on the same day (22nd or 23rd of September) but we name it differently
depending upon where we live. After that day, those of us in the Northern
Hemisphere can expect the days to continue to get shorter than the nights as we
move towards winter while the folks in the Southern Hemisphere
will notice that the days continue getting longer than their nights as summer approaches
them.
Weird. Are any other planets tilted?
Yes! As a matter of fact, all of them are tilted.
For example, Mars' axis is tilted 24 degrees from its orbital plane. Notice I said "from its orbital plane". Axial inclination is the amount of tilt the planet has with respect to its orbital plane - not the Earth's orbital plane. Perhaps you will recall from our lesson on orbits that each planet orbits the Sun in a plane that is not the same plane as the Earth's orbit. Mars' orbital
plane is at an inclination of 1.8o with respect to the Earth's orbital plane. That's pretty close to the Earth's orbital inclination, which is zero (by definition). It just so happens that Mars is tilted from its orbital plane only slightly more so than Earth. That means that Mars will have similar seasonal variations in the Sun's position. It will have its own Solstice and Equinoxes but they will be evenly distributed around the Martian "year" of 687 (Earth) days. (Another coincidence is the fact that Martian days are only 40 minutes longer than Earth days. I think Mars is just crying out for us to call it our second home! )
By the way, just because Mars has a tilt similar to the Earth's don't think that it points to Polaris. You have to take into account the way the orbital plane is set up and that adds a complication. In fact, the Martian North Pole points to within a few degrees of Deneb!
Venus has an axial inclination of 178o.
What? That's means it's upside down! How can a planet be upside down?
Well, that depends on how you measure it. You see all the planets (and most of the asteroids and comets) orbit the Sun counter-clockwise (as seen from a point high above the Earth's North Pole). This counter-clockwise motion was dictated by the direction of rotation of the primordial accretion disk from which all the planets formed. Venus, like all the planets, goes around the Sun counter-clockwise but it rotates in the opposite direction! When measured with respect to its orbital plane, Venus' axial inclination is 178o, just 2o from being perfectly upside down!
That means that on Venus the Sun rises in the WEST and sets in the EAST! (Sadly, if you were on the surface of Venus you wouldn't see the Sun through all the clouds anyway.)
We suspect that, some time long ago, a large asteroid struck Venus off-center, a glancing blow, that toppled it over. It must have been one heck of a whack!
If you think that's weird, you're right, but Uranus has an unusual tilt too. It has an axial inclination of 98o. That means Uranus is so bent over that it spins on its side! Technically, because its axial inclination is greater than 90o, Uranus also spins in the wrong direction. That too is weird but think about the seasonal variations on a planet with an inclination near 90o. One hemisphere points almost directly at the Sun during its Solstice while its other pole is hidden in darkness. The seasonal variations on Uranus would be enormous - if it had a solid surface. Only during the Uranus Equinoxes does Uranus have reasonable days of daylight and nighttime.
We assume that, like Venus, Uranus must have been pushed over by some kind of impact. Note, however, that Uranus is much, much larger than Venus, so whatever was responsible for pushing Uranus over must have carried a lot of power. This has to do with a property called "the conservation of angular momentum", a kind of rotational inertia. It would take a lot of force, called "torque", to tip Uranus over.
Pluto is a combination of both Venus and Uranus. Pluto has an axial inclination of 122.5o so it too is spinning the wrong way and it will have pretty excessive seasonal variations. (Very cold in the summer and very, very cold in the winter! ) Of course, we expect Pluto to be odd anyway because it's probably a captured moon. It probably didn't form from the accretion disk in its current position.
Mercury has an axial inclination of only 2o and Jupiter is tilted only 3o from its orbital plane. Therefore, they have very little seasonal variation between hemispheres.
Neptune's axial inclination is 28.8o, similar to that of Earth and Mars.
Saturn's tilt is also similar to the Earth's but Saturn's rings make the tilt more interesting and obvious.
Saturn has an axial inclination of 26.4o and its rings are pretty much aligned with its equatorial (NOT orbital) plane. That means as Saturn orbits the Sun, its rings (like the planet) will take on positions in which the "northern" face of the rings are in sunlight more than the "southern" face of the rings. This axial inclination also gives the rings important viewing qualities. Saturn's rings will appear from Earth to tilt as Saturn goes around the Sun. That means sometimes the rings will be obvious, when they are positioned facing us, and sometimes the rings will appear as nothing more than a thin line as we view them edge on. | ![]() |
The orientation of Saturn's rings are a clue to their origin. If they were along Saturn's orbital plane (near the ecliptic) we might assume they were derived directly from the accretion disk. That is, if they had formed from the accretion disk, the rings would be aligned with the Saturn's orbital plane - or pretty close to it. But they are not. The are tilted in such a way as to coincide with Saturn's equatorial plane. The best explanation for that alignment is that the rings formed recently from something orbiting Saturn, along its equatorial plane, that was broken into bits. Those bits now orbit Saturn along it equatorial plane making beautiful rings.
It is worth mentioning that each planet is maintaining its axial orientation as it orbits the Sun. Of course the Earth does this too. That's why our North Pole always points to Polaris. All the other planets will keep their polar axis pointing in a specific direction. They won't be pointing at Polaris. They will be pointing somewhere else. I don't even know if any of them have a nice "North Star" like we have. Regardless, I want you to understand that the axial inclination of all the planets is maintained as they orbit the Sun.
Well, that's the way things look from outer space, but what does axial inclination mean to those of us down here on Earth?
That's a good question. In the next lesson we will "come down to Earth" and consider how these things affect the path of the Sun through the sky. You will see that, by understanding the Sun's path, you can easily understand why the planets and stars do not follow simple paths along our Celestial Equator.