The ecliptic and the sidereal motion of the sun Moon and the planets on it. The following picture is a picture of the sky as it looks about noon on May 18 2012. The light of the Sun has been erased artificially via software. For that figure a commercially available planetarium program has been used. We also can see that Jupiter Venus and Mercury are close to the Sun. However we see that we can mentally connect the Sun and the three planets along a line (circle since we are on the celestial sphere). This mental line is called the ecliptic and it is a large circle on the celestial sphere. The ecliptic is passing from the 12 constellations of the zodiac. If you mentally draw the ecliptic you can see that it crosses Pisces, Aries, Taurus and Gemini from West to East. You can see that currently the Sun is entering Taurus, Mercury is in Aries and Venus is about to exit Taurus. In the figure I also show you the Stars Pleiades.
Now I will show you the same picture of the sky on May 30, 2012 as seen from Tampa once more at noon. The Sunlight has been erased via software. The constellation look the same, the planets and the Sun still lie on the same line the ecliptic. The ecliptic circle is always the same on the celestial sphere. However the Sun (and the planets) are not on their original place on the ecliptic line compared with the constellations. Observe that the Sun is now more to the left compared with the constellations of the Zodiac and the Pleiades. Since when I look at the Sun I am looking South the Sun is more to the East with respect to the fixed Stars 15 days later than my original photo of the sky. Now I will show you the same picture of the sky on June 16, 2012 as seen from Tampa once more at noon. The Sunlight has been erased via software.
The constellation look the same, the planets and the Sun still lie on the same line the ecliptic. The ecliptic circle is always the same on the celestial sphere. However the Sun (and the planets) are not on their original place on the ecliptic line compared with the constellations. Observe that the Sun is now exiting Taurus and about to enter Gemini. It always moves Eastwards with respect to the stars. The same for Jupiter and Mercury, but Venus happens now to be retrograde. It has shifted Westwards (which is not the usual motion) along the ecliptic with respect to the stars. Sometimes planets appear to be retrograde. All stars, the Sun, the Moon and the planets move from Eats to West (left to right in the Northern Hemisphere) due to the spin of the earth. This is the daily motion. However the Sun appears to be slower that the fixed stars and it is always left behind (to the East) with respect to them. The Sun is actually left behind one degree per day and one year later it will be in its original position with respect to the constellations having traversed all the points of the ecliptic. In antiquity the belief was that there are more than one celestial sphere each for every planet and a different deity was also residing in them. Today we know that this East-wise shift or slowing of the Sun along the ecliptic and the fixed stars is because of parallax, that is the change of the Earth -- Sun line of view as the Earth rotates around
the Sun. As the Earth rotates around the Sun that change of line of view aligns the Sun with different constellations and generates a picture of the celestial sphere where the Sun wanders around the ecliptic line. This is the annual motion of the Sun along the ecliptic and this motion is relative to the stars. This is a kind of backward motion from West to East. The Sun the Moon and the planets therefore as viewed from Earth they participate to two motions simultaneously. The daily motion due to the spin of the earth and they appear to make a full circle from East to West in 24 hours and the motion along the ecliptic relative to the other stars from West to East (backwards). That motion along the ecliptic is different for each planet, the sun and the Moon. For the Sun it takes a year to complete the ecliptic circle, for the moon one month and it is different for each planet. Planets some time appear to move along the ecliptic from east to west when they are retrograde. The Sun and Moon are never retrograde. It takes the Moon one month to traverse all the points of the ecliptic and all the constellations of the Zodiac. The shift of the moon along the ecliptic is also always East ward (like the Sun s although 12 times faster). This is how the phases of the Moon are formed. When the Moon and the Sun are together on the ecliptic it is New Moon. When the Moon has shifted 90 degrees eastward from the sun on the ecliptic it is first quarter moon, when 180 degrees full Moon, when 270 degrees third quarter Moon. Less than 90 degrees (before first quarter) waxing crescent, after first quarter and before full moon (between 90 and 180 degrees) waxing gibbous, after full moon and before third quarter (between 180 and 270 degrees) waning gibbous and after third quarter and before new moon (between 270 and 360 full circle) waning crescent. It goes: New Moon Waxing Crescent First quarter Waxing Gibbous Full Moon Waning Gibbous Third Quarter Waning Crescent New Moon. The corresponding East-wise angular distance from the Sun along the ecliptic circle on the celestial sphere is: 0 degrees less than 90 90 less than 180 180 less than 270 270 less than 360 360 or together with the Sun again The above relative position of the Sun and the Moon can help us find out what time the Moon rises or sets when it is at a particular phase. Example: Assume that we have a third quarter Moon? What time does such a Moon rise or set? We have third quarter of the Moon when the Moon is 270 degrees Eastwards from the sun along the ecliptic. It is the same as the Moon being 90 degrees westward along the ecliptic because the circle has 360 degrees. It is also the same as the Sun being 90 degrees
Eastward from the Moon along the ecliptic. When such a moon is rising at the East the Sun is 90 degrees eastward, that is it has not risen yet, it will rise 90 degrees later that is 6 hours later because it takes 24 hours for the whole ecliptic circle to rise. Therefore it must be midnight. What time doe such a moon (third quarter) set? When such a Moon set the sun is 90 degrees eastward that is at the zenith or 90 degrees (6 hours) before it sets. Therefore it is noon. Exercise: Repeat the same with the first quarter Moon. Hint: A first quarter Moon is 90 degrees to the East of the Sun along the ecliptic circle, that is the Sun is 90 degrees to the west of the Moon along the ecliptic. In order to visualize and see the eastward sidereal motion of the Moon on the ecliptic (eastward motion with respect to the stars) and how the phases of the moon are formed I will give you some photos of the sky on May 20, 22, 24, 26, 2012 and on June 17 and19 2012 one month later. We start from New moon and as the moon moves eastward from the sun it becomes waxing crescent until the 26 where it is a little less than first quarter. Observe how much faster than the Sun and the planets the moon is moving eastwards on the ecliptic with respect to the stars. The photos are always corresponding with the sky at noon. Please notice that the Moon Sun and planets participate to the daily (diurnal) westward motion as well like all the stars, on the top of their extra sidereal eastward shift along the ecliptic.
Notice that on May 20 2012 The Moon the Earth and the Sun are almost perfectly aligned and as viewed from the Earth the disk of the Moon almost covers completely the Sun. We may have an (annular) Solar eclipse.
On June 17, 2012 the Moon has almost completed the ecliptic circle and it moves eastward to meet with the sun once more. The next day we will have a new moon again. The Waning Crescent Moon is fairly visible in the sky.
On June 19 2012 we will have again New Moon but the Earth Moon and Sun are not now perfectly aligned. You can see that the disk of the Moon as viewed from earth is a little below from the Sun. This means that the moon is a little below the ecliptic plane and its shadow will miss the earth. That is why we cannot have an eclipse every New Moon (solar) and every Full Moon (Lunar).