Tag Archives: Earth

Lunar and Solar Eclipses of October 2014

Interestingly, there is both a lunar and a solar eclipse this month. The relationship between these events provides an opportunity gain a deeper understanding of eclipses, and it is an opportunity to explore some characteristics of the Moon’s orbit about the Earth.

Let’s start with the Sun’s role. The Sun’s path among the stars defines the ecliptic. The Sun’s location in the sky, and on the ecliptic can be computed rather precisely for any given date or time. The ecliptic can be found on most star charts. Note that the ecliptic becomes a full 360° circle when the left (west) and right (east) edges of a full sky chart are bent into a cylinder so that the two ends of the ecliptic meet.

A definition of Full and New Moon is essential to understanding solar and lunar eclipses. The Full Moon (the entire face of the Moon is lit) occurs when the Sun and Moon are opposite one another when seen from Earth. In other words, the Earth is located between the Sun and the Moon. The New Moon (none of the face of the Moon is lit) occurs when the Moon is located between the Sun and the Earth. The lit side of the Moon is facing the Sun, and the dark side is facing the Earth.

The Moon’s orbit is inclined to the ecliptic by about 5.5°, which means three things: 1) half of the Moon’s orbit is above the ecliptic, 2) half of the Moon’s orbit is below the ecliptic, and 3) the Moon crosses the ecliptic twice in each orbit. These two points are called nodes. The ascending marks the point at which the Moon crosses the ecliptic headed north, and the descending node marks the south-bound crossing. These nodes progress about the ecliptic once in about 18.6 years, which is why series of lunar and solar eclipses repeat ever 18.6 years.

If the Sun happens to be located at the point of the Moon’s crossing of the ecliptic at the time of the crossing, an eclipse will occur. Since the Sun’s disk (1/2° in diameter) occupies only about 1/720th of the 360° ecliptic, and the Moon may be as much as 5.5° above or below the ecliptic, an eclipse is a rather rare event.

A lunar eclipse occurs when the full Moon passes through the Earth’s shadow. Given that the Earth is between the Sun and Moon at Full Moon, it stands to reason that the Earth’s shadow will fall upon the Moon, if the full Moon happens to be crossing the ecliptic.

A solar eclipse occurs when the new Moon casts its shadow upon the Earth’s surface. This stands to reason given that at New Moon, the Moon is located between the Sun and the Earth. To an observer at a fixed location on the surface of the Earth, the Moon’s dark disk is seen to move across the Sun’s face, either partially, or fully blocking out the Sun at the eclipse’s maximum.

October 8th – Total Lunar Eclipse
This eclipse will begin when the Moon enters the prenumbra (lightest part of the Earth’s shadow) at 4:45am. The Moon enters the umbra (the darkest part of the Earth’s shadow) at 5:15am, and Moon is fully within the umbra (total eclipse) at 6:25am. Unfortunately, the Sun rises and the Moon sets before the eclipse ends.
http://en.wikipedia.org/wiki/Lunar_eclipse

October 23rd – Partial Solar Eclipse
This month’s solar eclipse is “partial,” because the Moon’s dark disk will not fully cover the face of the Sun. The eclipse will begin when the Moon first begins to cover the Sun’s face at 5:52pm, and it will reach its maximum coverage of the Sun’s face at 6:17pm, which is sunset.
http://en.wikipedia.org/wiki/Solar_eclipse

Aphelion – July 24, 2014

The Earth reaches aphelion (Greek apo [away from] + Helios [Greek god of the Sun]) on July 24th. Aphelion is the point in the Earth’s orbit that is farthest, or 94,555,000 miles from the Sun. Relatedly, perihelion (Greek peri [around] + Helios [Greek god of the Sun] is the point in the Earth’s orbit that is closest to the Sun, or 91,445,000 miles from the Sun.

Aphelion and perihelion apply not only to the Earth’s orbit about the Sun, but any object that orbits the Sun to include all of the planets, asteroids, comets, and even man-made satellites in solar orbit. Apogee and perigee are similar terms for objects that orbit the Earth, which are either the Moon or man-made satellites.

Aphelion and perihelion points are not only opposites that describe the Earth’s farthest and closest distance to the Sun during each annual orbit, these two points are on opposite sides of the Sun. Aphelion occurs July 24th and perihelion occurs on January 4th. Although the aphelion and perihelion points are on exactly opposite sides of the Sun from one another, the dates are not exactly six months apart. This is because the Earth moves more slowly during the perihelion to aphelion (up hill) part of its orbit, and faster while during the aphelion to perihelion (down hill) part of its orbit.

We learned in grade school that the Earth’s average distance to the Sun is 93 million miles. Those who remember that fact at all are likely, if asked, to omit “average,” and state that the Sun is 93 million miles away. If the Earth varies in distance to the Sun from 94.5 to 91.4 million miles over the course of each orbit, then that accounts for the average figure of 93 million miles that is often cited.

Let’s explore why the Earth’s distance to the Sun varies, and what the implications are of that variance. If the Earth were in a perfectly circular orbit, then its distance from the Sun would not vary. In this case, 93 million miles might be the constant distance to the Sun throughout each annual orbit. Like the orbits of most objects around a parent body, the Earth’s orbit is not circular, but is elliptical. An ellipse resembles a stretched or flattened circle, and an object following this type of orbit will vary between its closest and farthest points once per orbit.
http://simple.wikipedia.org/wiki/Aphelion

The Earth’s elliptical orbit has implications for solar eclipses. Both the Sun and the Moon are said to be about 1/2 degree in angular diameter as observed from Earth. This is because the actual diameter of both or coincidentally 400 times their diameter, and it accounts for the near-perfect fit when the Moon barely covers the face of the Sun during solar eclipses. The Sun’s apparent size when seen from Earth varies inversely with respect to Earth-Sun distance as the Earth moves from aphelion to perihelion. The Sun appears smaller when viewed from Earth at aphelion, and larger at perihelion. Similarly, the Moon appears smaller at apogee and larger at perigee. Consider the effect if the Earth were at perihelion and the Moon were at perigee at the time of a solar eclipse. The Sun would be at its smallest possible apparent diameter and the Moon would be at its largest possible apparent diameter. The result would be a total Solar eclipse where the larger Moon completely obscures the face of the smaller Sun. Now consider the opposite case. The smallest possible Moon at apogee would not be able to completely obscure the face of the largest possible Sun at perihelion. The result would be an annular Solar eclipse where the Sun would appear as a ring around the Moon.
http://simple.wikipedia.org/wiki/Solar_eclipse

Lunar Eclipse – April 15, 2014

The full Moon occurs when the Moon is at the point in its orbit on the side of the Earth opposite the Sun. Another way of describing this, is to say that the Earth is directly between the Sun and Moon when the Moon is full (fully lit). The Earth, like all objects upon which sunlight falls, the casts a shadow. This shadow extends into space in the Moon’s direction at full Moon. The Moon usually misses the Earth’s shadow by passing just a little above or below it at full Moon. But the Moon will pass through the Earth’s shadow this month in the early morning hours of April 15th. The Moon enters the penumbra (the lightest part of the Earth’s shadow) at 12:37am EDT, but the best viewing begins at about 2am when the Moon enters the umbra (the darkest part of the Earth’s shadow). Totality occurs when the Moon is completely inside the umbra from 3:06am until 4:27am, at which time it begins to re-enter the penumbra. This stage of the eclipse is the beginning of the Moon’s exit from the Earth’s shadow. The Moon completely exits the umbra at 5:30am, and the eclipse is completely over when the Moon exits the encumbrance at 6:30am.

There are two really neat things to note about the eclipse. First, the “sunset” effect. The portion of the Moon within the umbra will have a distinct reddish cast as a result of the Sun’s rays passing through the edges of the Earth’s atmosphere. As the eclipse progresses, the umbra can first be seen on the eastern edge of the Moon. Over the next hour or so, the portion of the Moon covered by the umbra will grow larger, until the Moon is completely engulfed by the umbra. The reverse will occur as the Moon slides back out of the umbra. Here’s the other neat thing to note. What the observer is actually watching as the Moon passes through the Earth’s shadow is the Moon moving along it’s orbital path around the Earth.