Ecliptic/Quizzes/Gallery

The purpose of this Gallery of images and explanations is to facilitate an understanding of the questions on the quizzes associated with the Wikipedia article.

Ecliptic and the zodiacal constellations

The celestial sphere is a map showing the positions of all the stars, which where generally assumed to remain "fixed" in ancient times. While this map is most faithfully represented on a sphere (globe), it can also be drawn as a rectangle using, for example, the equirectangular projection.

The annual path of the Sun against the zodiacal constellations

Great circles

A great circle is any circle that divides the sphere in two equal hemispheres. It is the largest circle that can be drawn on a hemisphere. The celestial equator and the ecliptic are two important great circles on the celestial sphere. Another important great circle is the path of the moon taken in the course of one month.

Geocentric perspective of lunar eclipses

Shown are two great circles on the celestial sphere. Each represents the path of the Sun or Moon against the background ("fixed") stars. A lunar eclipse occurs when the Sun and the Moon appear at opposte nodes (N₁ and N₂).

1. The Sun travels around a great circle called the ecliptic each year, moving approximately 1° each day. The ecliptic remains nearly stationary on the celestial sphere for several centuries (making a full precession in 26,000 years).
2. The Moon travels around a different great circle each month, moving between 11.6° and 14.8° each day; (depending on how close the Moon is to the Earth). The Moon's path against the background stars evolves much more quickly (making a full precession in 18.6 years). A consequence of this precession is that the cycle two eclipses seasons takes only 346.6 days. In other words, in the time it takes the sun to complete its annual path around the celestial sphere, the moon's path has noticeably changed and has to be "redrawn".

A solar eclipse occurs when the Sun and Moon occupy the same place on the celestial sphere. A lunar eclipse will occur only when the Sun and Moon occupy opposite points on the celestial sphere. This can only occur when both objects are on one of the nodal points (N₁ and N₂).

How to remember the relative motion of the Stars, Sun, and Moon

There are a number of ways to remember things. Mnemonics are the most popular, but an educated person should attempt to use more rigorous methods because they lead to an understanding of the situation that can be applied to related problems.

Take the limiting case

The best way to remember whether the Sun, Moon, or Stars rises and sets the fastest is to imagine that the 24 hour rotation of the earth is different.

1. For example, if the earth took an entire year to rotate about its axis, the sun would always be in the same place, while the stars take a year to rise or set. This informs us that the stars rise and set faster than the Sun does. As you gradually increase the rate of Earth's rotation about its axis, this trend will be diminished, but it is a good guess that stars will always appear to rise and set at a faster rate than the Sun.
2. If the earth took 24 hours to rotate, the Moon would be stationary. Therefore the Moon is also slower than the Stars.
3. So which is slower, the Sun or the Moon in the sky? It takes a less drastic change in Earth's rotation to "stop" the Moon, which suggests that the Moon is the more extreme of the two objects in the sky.

This method is known as "taking the extreme limit" is a good starting point for making an educated guess. It tends to work with simple systems, but may fail for complex systems, especially biological systems. For example, an overdose of vitamin E will kill you. But that does not mean that vitamin E is bad for your health.

Mnemonic: Start a Sunny Month

The stars are the fastest, then the Sun, and finally the Moon is slowest. It also helps to incorporate the previous idea: Standing on a rotating Earth is like sitting in a car, passing a moving Moon (which has an angular rotation that is 30 times slower). The motion of the Earth causes the apparent motion of the Moon to slow down.

Make a sketch

The diurnal motion of zodiacal stars is faster than that of the Sun's along the ecliptic. The Moon's diurnal motion is the slowest.

Imagine that you are in the northern hemisphere, facing south, with the eastern horizon to the left and the western horizon to the east. Draw a couple of stars, traveling west with a large arrow, the sun with a smaller arrow, and the moon the slowest. Practice the quiz a couple of times, and almost all student finally will get it right.