Understanding Astronomy: The Sun and the Seasons
The apparent path of the Sun's motion on the celestial sphere as seen from Earth is called the celestial equatorial plane along the line between the equinoxes. Precession of the equinoxes, motion of the equinoxes along the ecliptic (the plane of The celestial equator is inclined at a ° angle to the ecliptic (the. in the drawing to the left; the difference between the equatorial and polar Precession also causes the plane of the Earth's equator to wobble, which ( along with Pisces, Libra, and Sagittarius) quarters the ecliptic, Gemini is still traditionally taken as the Solstice's celestial home.
Celestial Coordinate System
For obvious reasons, we define our day based on the motion of the sun, not the stars. Moreover, the location of the sun's path across the sky varies with the seasons, as shown in the computer-generated image below, which shows the eastern sky, viewed from a mid-northern latitude.
- Precession of the equinoxes
- PHY115: Professional Skills in Physics and Astronomy
- The Sun and the Seasons
This simulated multiple-exposure image shows the path of the rising sun through the eastern sky on the morning of the 21st of each month, from December at the right through June at the left. The spreading of the trails as they go upward is a distortion caused by stretching the domed sky onto a flat semicircle.
MEASURING THE SKY
The sun's path through the rest of the sky is similarly farther north in June and farther south in December. The sun appears to move along with the celestial sphere on any given day, but follows different circles at different times of the year: At the equinoxes, the sun's path follows the celestial equator.
In late March and late September at the "equinoxes"the sun's path follows the celestial equator. It then rises directly east and sets directly west. The exact dates of the equinoxes vary from year to year, but are always near March 20 and September After the March equinox, the sun's path gradually drifts northward. By the June solstice usually June 21the sun rises considerably north of due east and sets considerably north of due west. For mid-northern observers, the noon sun is still toward the south, but much higher in the sky than at the equinoxes.
After the June solstice, the sun's path gradually drifts southward. By the September equinox, its path is again along the celestial equator. The southward drift then continues until the December solstice usually December 21when the sun rises considerably south of due east and sets considerably south of due west.
For mid-northern observers, the noon sun is quite low in the southern sky. After the December solstice, the sun's path drifts northward again, returning to the celestial equator by the March equinox. At the equinoxes, exactly half of the sun's circular path lies above the horizon.
But notice that in June, considerably more than half of the circle is above the horizon, while in December, much less than half the circle is visible.Basics of Astronomy: The Celestial Sphere
This is why, if you live in the north, you have more hours of daylight in June during your summer than in December during your winter. The Seasons The added hours of daylight are one reason why summer is warmer than winter. But there's another reason that's even more important: Notice from the illustrations above that the noon sun is much higher in June than in December. This means that the sun's rays strike the ground more directly in June. In December, on the other hand, the same amount of energy is diluted over a larger area of ground: The intensity of sunlight striking the ground depends on the sun's angle in the sky.
When the sun is at a lower angle, the same amount of energy is spread over a larger area of ground, so the ground is heated less. There is a common misconception that summer is warmer than winter because the sun is closer to us in the summer.
Actually the sun's distance hardly changes at all—and in fact, the sun happens to be closest to us in January. Again, the seasonal changes in climate are caused by the varying angle of the sun's rays, together with the varying amount of time that the sun is above our horizon.
The Sun on the Celestial Sphere Although we never see the sun and the stars at the same time, it's not especially hard to figure out which stars and constellations the sun is lined up with on any given day: Just look at the constellations in the east a little before sunrise, or the constellations in the west a little after sunset, and allow for the angle of the sun below your horizon.
The ecliptic is a great circle on the celestial sphere, tipped Its orientation with respect to our horizon changes as the sphere spins around us each day. It has the orientation shown here at noon in December and at midnight in June. If you plot the sun's daily location on a star chart or celestial globe, you'll find that it gradually traces out a great circle, called the ecliptic. So the ecliptic is an imaginary circle around the celestial sphere, centered on us, that marks all the possible locations of the sun with respect to the constellations.
Here, it is shown projected outward gray to the celestial spherealong with Earth's equator and polar axis green. The plane of the ecliptic intersects the celestial sphere along a great circle blackthe same circle on which the Sun seems to move as Earth orbits it. The intersections of the ecliptic and the equator on the celestial sphere are the vernal and autumnal equinoxes redwhere the Sun seems to cross the celestial equator.
Because Earth's rotational axis is not perpendicular to its orbital planeEarth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. Axial precession astronomy The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26, years, a process known as lunisolar precessionas it is due mostly to the gravitational effect of the Moon and Sun on Earth's equatorial bulge.
Likewise, the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as planetary precession.
The combined action of these two motions is called general precessionand changes the position of the equinoxes by about 50 arc seconds about 0. Astronomical nutation Once again, this is a simplification.
Periodic motions of the Moon and apparent periodic motions of the Sun actually of Earth in its orbit cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as nutation. It is about Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.
Obliquity of the ecliptic for 20, years, from Laskar The red point represents the year Northern hemisphere days are now the longest of the year, nights the shortest, the extent of the effect dependent on latitude. Conversely, following the autumnal equinox, as the Sun moves south, it rises and sets progressively farther south of east and west. Northern hemisphere days now get shorter less than 12 hoursnights longer greater than 12 hours.
On December 22, the Sun reaches its most southerly extent see the left-hand "Earth"at a declination of It then rises as far south of east and sets as far south of west as possible. Northern-hemisphere daytime is now minimized, nighttime maximized. All the effects are reversed in the southern hemisphere, while at the Earth's equator, days and nights are always equal at 12 hours.
When at the Summer Solstice, the northern-hemisphere Sun north of the tropics crosses the celestial meridian as high as possible, while at the Winter Solstice it crosses as far to south as possible.
In the summer, sunlight spreads itself over a smaller area of ground than it does in winter, and thereby heats the ground more efficiently, yielding more heat, so it is hot in the summer, cold in the winter. This effect is the sole cause of the Seasons. The effect of the variation in distance between the Earth and the Sun caused by the Earth's elliptical orbit is of little consequence because of the power of the oceans to store heat.
Above the Arctic Circle at latitude