THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
"A season seems a mystical fairy world when you're not currently experiencing it."
THE DAILY ASTRONOMER
July 1, 2014
July 2014 Night Sky Calendar
July fascinates us. First, it is the only month of the year when we face no prospect of snow whatsoever. Secondly, the nights are frustratingly short, but delightfully warm, thereby proving that fleeting bliss is preferable to no bliss at all. Third, the night length is gradually increasing as the Sun's altitude decreases, but this decrease is so slight that unless you're one of those nutcases obsessed with astronomy, you won't notice much difference. One notices the converse in January, when the daylight duration increases a bit each day, but we're locked inside our houses weeping with despair, so we pay scant attention. Though the night sky calendar focus generally relates to solar system objects, it behooves us to mention that the Summer Triangle rises into greater prominence now and will remain a beautiful evening sky sight until early winter. With this triangle rises the rich Milky Way Galaxy star field: the region surrounding the unobservable nucleus. The Scorpion, Archer and Serpent Charmer -Scorpius, Sagittarius and Ophiuchus, respectively- ride along the southern evening sky; the Lion meanders toward the west; the flying horse appears in mid evening; and amongst this menagerie, we find the restless planets, phasing moon, and more than a scattering of meteors. Oh, if only the nights were longer…forget we said that.
TUESDAY, JULY 1: MERCURY STATIONARY (RESUMES EASTWARD MOTION)
Every month we remind subscribers that the planets don't change course. Every planet moves in the same direction, and they have been ever since they formed out of the nebula. Generally, the planets appear to become stationary and alter direction because we're observing them from a moving platform (i.e. the revolving Earth.) For instance, when we pass the slower moving superior planets -those worlds farther from the Sun-, they appear to move backward. However, the inferior planets would exhibit direction changes even if we watched them from an unchanging location. The orbits of the two inferiors -Mercury and Venus- are inside Earth's and, like fans watching the Indianapolis 500, we could see the planets rapidly spin through their entire orbits were the Sun not in the way. We'd see the inferior planets move in one direction when they traveled between Earth and the Sun; and would revolve in the opposite direction when they're on the far side. They would change directions at either end of the orbit. Tonight, Mercury's retrograde motion stops and it will resume prograde motion.
WEDNESDAY, JULY 2 MIDDAY: MID POINT
Just a little calendar reckoning. July 2st is the 183nd day of the year; and we have 182 days remaining. So, we've burned though the first fifty percent of 2014 and the next half remains not yet lived, just to mix our metaphors. The actual time is about 12 UT, or 12:00 p.m. Greenwich Time. The mid point of each year is July 2 at 12:00 p.m. on regular years; July 2 at midnight on leap years. (The next leap year is 2016)
July is named after Roman dictator Julius Caesar, who tried to subdue the world and might well have succeeded had he not trusted his friends so implicitly. Amongst the myriad ways he transformed the world was in his calendar reformation. The Julian calendar, also named for him, defined a year as consisting of 365 days with an extra day every four years. Centuries after the calendar's adoption, the gradual equinox shift displaced it from March 21st to March 11.* This stark discrepancy necessitated Pope Gregory XIII's 16th century calendar reform, as the 325 Council of Nicea's definition of Easter related directly to the vernal equinox. The new calendar reform, called Gregorian, was introduced in 1582. In this corrected, and more accurate, version, every year equally divisible by four is a leap year, except for century years not equally divisible by four. (For instance, 1900 was not a leap year; 2000, however, was a leap year. That was only the second century leap year we've had since the Gregorian reform: a fact that we still consider fantastic.)
THURSDAY, JULY 3: EARTH AT APHELION
English purists balk when people say, "Earth circles the Sun," because 'circle' is a noun, not a verb. Another objection, less often cited, but no less important, is that Earth's orbit is not circular, but elliptical. A circle is a closed curve with all points equidistant from a common center. An ellipse is a close curve with two foci (pretentious plural of focus) instead of one center. One may envision this curve as an oval, however, some ellipses, such as that defining Earth's orbital path, are so slight that they appear circular. Were Earth's orbit a perfect circle, our planet would maintain a constant distance from the Sun throughout the year. As its path is elliptical, its distance continually changes, veering from a maximum distance (aphelion) to a minimum distance (perihelion.) Earth reaches aphelion today, and it will be 94,518, 574 miles from Sol. Compare this value to its mean distance of 92,955,070 miles. Though this difference amounts to more than a million miles, it represents a 1.6% percentage increase. This difference scarcely affects our temperature, but it does prolong summer! When Earth is closer to the Sun, its orbital velocity is greater. So, in January, when Earth reaches perihelion, it is revolving around the Sun after than it is in July, at aphelion. So, the amount of time Earth needs to travel along the quarter arc representing winter is less than the time Earth requires to move along the summer quarter arc. Summer is a couple days longer than winter, which is why the autumnal (September) equinox can occur anytime between Sept 22 - 24 and not on September 21.
SATURDAY, JULY 5: FIRST QUARTER MOON
SATURDAY, JULY 5: MOON 0.24 DEGREES NNW OF MARS (SILVER EVENT!!)
So close! We're not quite in the right spot to witness a lunar occultation of Mars. Were we in Northern South America, we'd observe the Moon pass directly in front of the fourth planet. As the moon lacks an atmosphere, when it moves in front of a star or planet, the occulted body snaps out of sight in an instant. If the moon had an atmosphere, the planet or star would slowly vanish as the Moon moved across it.
Alas, we will only see the quarter moon and Mars about a quarter of a degree apart. A beautiful coupling of moon and world that one can see from nightfall to just after midnight.
TUESDAY, JULY 8: MOON 0.44 DEGREES SSW OF SATURN (BRONZE EVENT!)
Yet another missed occultation. This time, observers in southernmost South America would see the Moon move in front of Saturn. Of course, we'll still watch the gibbous Moon and the Ring Lord moving hand in hand across the night sky. A beautiful sight that stays up most of the night. We conferred the bronze on this event and the silver on the Mars close approach because the separation distance is smaller.
SATURDAY, JULY 12: FULL MOON
Every full moon has a nickname. These names are often associated with the month. For instance, in winter we'd have the long night moon, the frost moon, or the snow moon. July's full moon is often called the Full Thunder Moon, as many violent thunderstorms occur in July. As plants flourish in July, it is also called the "Full Hay Moon." The Cherokee call it the "Ripe Corn Moon," but the Chinese refer to the July full moon as the "Hungry Ghost Moon." (Yes, we're really curious, too.)
SATURDAY, JULY 12: MERCURY AT GREATEST WESTERN ELONGATION
We'll never see Mercury at midnight. The first world remains far too close to the Sun for us to ever find it at the witching hour. Instead, Mercury oscillates between horizons. When it is at western elongation, we find it in the eastern pre-dawn sky. When at eastern elongation, Mercury lurks around the western early evening sky, Find Mercury low in the eastern pre-dawn sky this morning. It wil be 20.9 degrees west of the Sun and will be barely visible before the brightening twilight obscures it.
WEDNESDAY, JULY 16: MERCURY 6.2 DEGREES ESE OF VENUS (GOLD EVENT!!!)
"Gold event" refers to that single event of the month that one should at least try to observe. We realize that in a world lousy with the sensible and well adjusted, there will be many people hardly inclined to venture outside. Even those uninterested many might want to sneak a glance at the gold event. Mercury and Venus gather in the early morning eastern sky. Distinguishing between them won't prove difficult as Venus (magnitude -3.9) will be 33 times brighter than Mercury (-0.1), Remember that Venus leaves us soon and won't return until the very end of the year. Now is the time to find our sister planet before its prolonged hiatus. (Read more about Venus in the PLANET WATCH.)
FRIDAY, JULY 19: LAST QUARTER MOON
SUNDAY, JULY 20: SUN ENTERS CANCER THE CRAB
We often say that the Sun moves through thirteen constellations each year. It does nothing of the sort. Instead, Earth moves around the Sun and it therefore appears to migrate through thirteen constellations. We refer to this retinue as the "Zodiac," or "Ecliptic Band." Today, we see the Sun cross into the Cancer the Crab region after having left Gemini the Twins. The thirteen "zodiac" constellations are CANCER THE CRAB; Leo the Lion; Virgo the Maiden; Libra the Scales; Scorpius the Scorpion; Ophiuchus the Serpent Charmer; Sagittarius the Archer; Capricornus the Seagoat; and Aquarius the Water Bearer, Pisces the Fish; Aries the the Ram; Taurus the Bull and Gemini the Twins. The Sun moves into Leo the Lion on August 10th.
MONDAY, JULY 21: SATURN STATIONARY
Remember earlier we discussed planetary motions and how they appear to stop and reverse course when, in fact, they do nothing of the sort? This strange motion relates to the combined motions of Earth and the other planets. A superior planet -one more distant from the Sun than Earth- exhibits retrograde motion just before, during, and just after Earth passes between it and the Sun, called "opposition." The more distant planets revolve around the Sun more slowly than Earth, in accordance to Kepler's Second Planetary Law.** As Earth overtakes the superior planet, the latter will appear to move backward relative to the stars. When Earth moves on, the superior planet will eventually appear to "stop," and then resume prograde motion. In March, Saturn became stationary before beginning retrograde motion. On May 10, Saturn was at opposition. Now, Saturn has become stationary again and will start its prograde motion until early next year, when Earth approaches Saturn again. Note: Prograde motion is eastward. Each night, a planet moving in a prograde fashion migrates toward the east. When in retrograde, planetary motion is westward.
MONDAY, JULY 21: MOON 7 DEGREES S OF THE PLEIADES
The Seven Sisters are back, but visible only in the early morning. We like to think of them as the mythical coven of Celtic sorceresses that appear momentarily in moonlit wields when nobody is around to observe them. Of course, that analogy falls apart because, well, one can actually see the Seven Sisters provided that one ventures out at a few hours after midnight to behold them in all their loveliness. Tonight, they appear to veer close to the waning crescent moon. Although, the moon will still resemble a quarter more than a crescent tonight.
THURSDAY, JULY 24: MOON 4.4 DEGREES SOUTH OF VENUS
The crescent Moon-Venus gathering is always beautiful! In fact, the Moon has to be in a crescent phase whenever Venus is close to it, because Venus, being an inferior planet, never strays far from the Sun. As the Moon is always in a crescent phase when it is near the Sun, it will be a crescent when veering close to Venus. The Moon will have one more encounter with Venus (August 24) before the planet vanishes until late December. However, they'll be less than 20 degrees from the Sun and exceedingly difficult to find. This Moon-Venus gathering will be the last easy one to see for the rest of the year. (Venus and the Moon are together in the December 23rd evening sky, but, again, are not easy to see.)
THURSDAY, JULY 24: JUPITER IN SUPERIOR SOLAR CONJUNCTION
Nothing to see here! Jupiter is on the far side of the Sun relative to Earth and is not visible. In fact, July is not Jupiter's month. (See Planet Watch if you want to see that sentence repeated.) When a planet is on the Sun's far side, it is said to be in "superior solar conjunction." When a planet is between the Sun and Earth, it is in "inferior solar conjunction." The superior planets, those more distant from the Sun than Earth, can only be in superior solar conjunction. The inferior planets, Mercury and Venus, can be in both.
SATURDAY, JULY 26: NEW MOON
Beginning of lunation cycle 1133.
TUESDAY, JULY 29: MERCURY AT PERIHELION
Cast your mind back to the July 3 entry when we droned on incessantly about the shapes of planetary orbits. They are not circular, but elliptical, or, like ovals. Every planetary orbit is elliptical and therefore each planet's distance from the Sun constantly changes. These distances range from the greatest, aphelion, to the least, perihelion. On July 3rd, Earth reached aphelion and then started moving closer to the Sun. Today, Mercury is at perihelion, its closest distance, approximately 28.6 million miles. Compare this value to its average distance of 35.9 million miles. The difference is quite high, owing to Mercury's more elongated orbit.
MERCURY is a morning sight for the first part of July. It reaches greatest western elongation on July 12th, but will remain low in the pre-dawn sky even then. Mercury vanishes by late July, only to return to the early evening sky in mid August. VERDICT: If you must see Mercury, wait until the second week of July and venture out an hour before sunrise.
VENUS will be gone by late August and won't show its comely face at all throughout the autumn. (Yes, we've mentioned that once or thrice.) Venus lovers should seek it out this month as it is descending each night and by mid August will be quite difficult to find. VERDICT: the sooner in the month, the better. One can still enjoy the bright phosphorous goddess world above the morning twilight throughout July, but its highest early in July.
MARS: (PICK PLANET!) Yes, we picked Mars as the pick planet because the pickings are thin! Mercury and Venus are pre-dawn worlds only; Jupiter is a ghost; and Saturn is diminishing in brightness. In fact, we chose Mars as the pick planet because it is slightly brighter than Saturn. Granted, Mars and Saturn will be equally bright by month's end. VERDICT: A moderately bright red eye in the summer sky. Always distinctive because of its reddish color.
JUPITER: July is not Jupiter's month. After making a spectacle of itself throughout the winter and spring, Jupiter has finally been given a red card and kicked off the pitch. It is due for its profanity laced tirade against FIFA any minute, just to expand a metaphor beyonds the bounds of absurdity. VERDICT: Well, nothing to see here. We will wait for Jupiter to return to the pre-dawn sky in August. Unlike Venus, Jupiter's hiatus is brief.
SATURN shares the evening stage with Mars and is almost as bright, though not quite. Watch Saturn draw closer to the Sun throughout the summer and autumn. (It disappears in November.) VERDICT: Still a moderately bright evening sky sight. Though not brilliant, it is still quite easy to find provided one looks toward Libra, which has been its host constellation since September 2013 and will continue to be until January 2015.
PLANETS IN DECREASING ORDER OF BRIGHTNESS
Venus - Mercury - Mars - Saturn
(Note: Mercury begins the month dimmer than Saturn, but then becomes brighter before its disappearance.)
*Quaint fact # 1: the Vernal Equinox never occurred on March 21st in the medieval period. Quaint fact # 2: Nobody has ever agreed as to the medieval period's actual time frame, so one could legitimately dispute fact # 2.
**Kepler's Second Law of Planetary Motion, paraphrased for an attempt at clarity. "A planet's orbital velocity relates directly to its distance: the closer a planet is to the Sun, the faster it moves."
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- Southern Maine Astronomers Meeting: June 2, 2016 7:00 p.m.
- "Radio Astronomy" radio program: June 3, 2016 "Meteorites Part II"
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