Category Archives: Events

Geminids Meteor Shower – December 13-14, 2014

Look for this meteor shower just before midnight on these two nights. There is a good chance of seeing a hundred or more meteors per hour! The radiant (the apparent point of origin) is in the constellation Gemini, which will be high overhead as midnight approaches. Be sure to get some observing in before the Moon rises at about 11pm and midnight on these two nights.
http://en.wikipedia.org/wiki/Geminids

Leonid Meteor Shower – November 17-18, 2014

Look for this meteor shower at around midnight on these two nights. The radiant (the apparent point of origin) is in the constellation Leo, which will be high overhead at midnight. Interestingly, at any given time of the year, the Earth is moving toward whatever stars happen to be directly overhead at midnight, and it “smashes” into the debris field that crosses the Earth’s orbit and creates the shower.

The Leonid shower of 2002 was the best meteor shower that I have ever witnessed. I went out at around midnight near the peak, and the origin was nearly directly overhead. I could see about six to eight meteors per minute that seemed to fly out of the origin in all directions. While lying on my back and looking straight up at the origin the visual effect was very much like the old Windows screen saver that gave the appearance of flying through a star field.
http://en.wikipedia.org/wiki/Leonids

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

Supermoon – August 10, 2014

The largest full Moon of the year occurs at about 1:30pm on August 10th. The Moon does not rise above the horizon until just before 7pm, but it is still very nearly full at that time. A supermoon is said to occur when the Moon is full at the same time it is near perigee, or when it is at its closet approach to Earth during its orbit.
http://en.wikipedia.org/wiki/Super_Moon

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

2014 Summer Solstice

The Summer Solstice occurs this year on June 21st at 6:51am ET. Although it occurs at a specific instant in time, there is nothing that is easily observed directly, but there are some indirect observables worth noting. I will use this discussion as an opportunity to explain the four seasons’ relationship to the ecliptic.

The neatest star chart that I could find is a .jpg of one that was flown on the Apollo 11 mission. Please use it as a reference for the discussion that follows.  http://www.hq.nasa.gov/alsj/a11/A11StarChart-S1.jpg

Before proceeding, let’s locate a few key points on the chart. First, locate the equator, which is the 0° straight horizontal line across the center of the page. Note that at the left and right ends of this line are labeled “Vernal Equinox.” The Vernal Equinox, or Spring Equinox, is actually only one point, but since the equator is a circle that closes on itself, it is displayed here as two points. Next locate the ecliptic. This the sine wave that begins at Vernal Equinox at the left end of the equator and rises above it to about 23° before sloping back down. This highest point on the sine curve is the Summer Solstice, which is not labled on the chart. Proceeding toward the left past the Summer Solstice the ecliptic curves down toward and crosses the equator at mid-page. This point is called the Autumnal Equinox. Continue following the ecliptic to the left until it reaches it’s lowest point, which is called the Winter Solstice. Beyond this point, the ecliptic slopes back toward the north, reaching the equator again at the Vernal Equinox, thus completing the circle. Now go back to the beginning of the ecliptic at the left side of the page. Follow it to the left again, this time noting how many of the Zodiacal constellations you can find. I see nine of them, so three of the twelve constellations are not represented.

I have mentioned the ecliptic as the imaginary line among the background stars that marks the Sun’s path among them. In other words, if the Sun were just an ordinary bright star, we could see it and the background stars at the same time. If one plotted the Sun’s daily position on a star chart for a year and connected the dots, this line would represent the ecliptic. If this term is reminiscent of eclipse, it is and there’s a reason. If the Moon’s path, for instance, crosses the ecliptic at the point on the ecliptic where the Sun happens to be on that day, then there is a solar eclipse.

The equator on a star chart represents all of the points on the celestial sphere that would be directly over one’s head at all of the equatorial points on the Earth’s surface. If we took the star chart upon which we plotted the ecliptic and taped the left and right edges together with the stars on the inside and the two ends of the equator aligned, we would notice that the ends of the ecliptic are also joined. This is because the ecliptic is also a circle. I should mention that I placed the stars on the inside of the circle, because this is representative of our Earth-bound view from the center of the celestial sphere looking outward. We should note that while the equator stays centered between top and bottom of the circular chart, the ecliptic appears as a sine wave that crosses the equator twice while extending to a peak north of the equator and a trough south of the equator. The reason for the sine form is that the circle of the ecliptic is inclined 23 1/4 degrees to the circle of the equator. And to explain even further, this arrangement of the two circles occurs because the Earth’s axis is inclined to its orbit by 23 1/4 degrees. This was explained to us in grade school as the reason for the seasons that we experience, but few of us are really quite sure why this is so.

There are two points where the ecliptic crosses the equator, and there is a peak and there is a trough that all have special significance with respect to the four seasons. The point at which the ecliptic crosses the equator going from south to north is the Sun’s location on the first day of Spring, or March 21st, give or take a day. This is more properly called the Vernal (Spring) Equinox. The other crossing, where the ecliptic is crossing the equator from north to south marks the Sun’s location on the first day of Fall, or roughly September 21st. This is called the Autumnal Equinox. The point farthest north of the equator represents the Sun’s location on the first day of Summer, or roughly June 21st. This is called the Summer Solstice. And finally, the southernmost point is the Winter Solstice, which occurs on roughly December 21st.

Let’s consider what we know about the four seasons, and then examine how that relates to the ecliptic’s relationship with the equator. We know that the days are longest in the summer and that the Sun is higher in the sky near noon in the summer. Indeed the longest day of the year occurs with the Summer Solstice, which is when the Sun is on the northern most point of the ecliptic, and its rays shine more directly down upon our northern hemisphere location, thus creating Summer’s hot weather. The opposite is true for the Winter. We associate winter with shorter, colder days. The Sun is on the point of the ecliptic that is farthest south of the equator. The days are shorter and the Sun remains low in the sky at noon. The weather is colder because the Sun’s rays shine down on us at a less direct angle. What about the equinoxes? At the equinoxes, the Sun is directly over the equator, and the days and nights are of roughly equal length. The Spring and Fall are associated with the equinoxes, and the weather tends to be milder at this time.

So, we have explored the ecliptic’s relationship to the equator, and how the Sun’s position on the ecliptic is related to the seasons. We’ll save for another month an explanation of how this affects the Moon and planet’s position in the night sky.

The Moon’s Planetary Conjunctions During May 2014

When two Solar System objects arrive at their closest approach to one another as viewed from Earth, they are said to be in conjunction. This month I will examine the Moon’s close approaches to all five of the visible planets that were known to the ancients. As both the Moon and the planets are in constant motion, the actual conjunction is represented by an instant in time. Because of their slow apparent motion, the close approaches (visits?) can be observed for many hours before or after a conjunction.

As previously mentioned, the planets and the Moon never wander far from the ecliptic. One implication of this fact is that as the Moon completes its 28-day orbit around the Earth, it will be in conjunction with each of the planets once. This month, the young (thin) crescent Moon will first visit Jupiter near the western horizon in Gemini on May 3rd (closest) and 4th. Try to observe on both evenings and note that the Moon has moved eastward. Also note Jupiter’s position among Gemini’s stars, perhaps by making a sketch of Gemini that indicates Jupiter’s position. This sketch will come in handy near the end of the month.

Next up is a very interesting series of close encounters with three bright and colorful objects (Mars, Spica and Saturn) in the east at dusk on May 10th through the 14th. There are lots of things to observe over the course of these five evenings. First, the waxing gibbous Moon will grow larger each evening until it reaches full Moon on May 14th. Next, note that its location is a little farther east each evening. These two phenomena are the result of the Moon moving along its orbital path around the Earth, which changes its angle relative to the Sun. Also note that the point at which the Moon became full last month was closer to Mars (read about the lunar eclipse in April’s Scope Out), and this month the full Moon occurs closer to Saturn. This eastward slide of the full Moon from one month to the next happens because of the Earth moving along its orbital path around the Sun. And finally, note the distinct colors of the three objects: Mars is red, Spica is blue, and Saturn is yellow. The Moon will be near Mars on May 10th, and between Mars and Spica on May 11th. It will be between Spica and Saturn, but closer to Spica on the 12th, and closer to Saturn on the 13th. And finally it will be on the eastward side of Saturn on May 14th, the last evening of this string of encounters.

Another rewarding and challenging opportunity to observe the Moon arrives near month’s end as it transitions from a thin waning crescent in the eastern sky at morning, to a thin waxing crescent in the evening sky in the evening. First, observe the Moon as a thin waning crescent on the eastern horizon during its close encounter with Venus just before sunrise in the pre-dawn hours of May 25th. A careful observer might see an even thinner crescent very low on the horizon and closer to the sunrise point the next morning. After this, the Moon cannot be seen because it is lost in the Sun’s glare as it approaches new Moon (conjunction with the Sun) on May 28th. A young Moon (thin waxing crescent) emerges from the Sun’s glare on May 30th, and can be seen very low on the western horizon near Mercury. On the next evening, it will appear a little higher above the horizon, and it will once again visit Jupiter. Check the sketch that you made at the beginning of the Month. Has Jupiter moved among the stars since its last visit with the Moon on May 3rd and 4th?