J07T
SpaceWalk
FOR JUNE 2007
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John Pazmino
National Space Society
New York City Chapter
nyskiesastronomy@earthlink.net
I got a lot of good comments on the little chemistry lesson in the May SpaceWalk, which was a spinoff of the fictional mineral kryptonite. However, you noted: 'You didn't talk about the periodic table of elements.
Alright, I will, but just a little bit, OK? Recall that the atomic or element number is the number of protons in the atom's nucleus. This count defines the atom as one of a this or that element. Neon IS neon BECASUE it has 10 protons in its nucleus. That's why we don't usually write the proton count with the element symbol. Chemists -- and, increasingly, spacefarers and astronomers -- know by heart most of the important elements.
We can tabulate the elements in order of atomic number, along with some of their properties. This is how we tabulate the planets, stars, rocket boosters, and many other items of astronomy and spacefaring.
Chemical elements are a bit different. In the mid 1800s, Mendeleyev (spelling varies) discovered that elements seem to fall into groups of similar properties or of increasing or decreasing values for certain parameters. At the time, there were many gaps in the sequence of atomic numbers because many elements hadn't been isolated yet. Yet enough were in hand to bring out the pattern.
More than that, Mendeleyev found that a gap indicated that there should be an element and he predicted what its parameters should be. In, yes, in time they were found and they did have about the properties he stated for them.
To better organize the elements in a graphical form, Mendeleyev, and others, ranked them in rows and columns such that the chemical parameters smoothly varied from right to left, from top to bottom. The columns are 'groups'; rows, 'periods'. This graphical layout became the modern Periodic Table of Elements.
In text form, it's a bit clumsy to draw a decent periodic table, but just about any textbook on science, particularly chemistry, has one. I sketch one here to get you started.
~See 1st table to the left~
There is no standard content for a periodic table. All have at least the element symbol and element number, which is all I give in the sketch above. The symbol is a single or dual letter chosen to avoid duplications. On the whole, the older recognized elements got the single letters. Since the mid 20th century all new elements receive dual-letter symbols, even if a single letter causes no conflict with existing symbols. You just have to know the symbols; some are not obvious.
Most tables give the mass number of the element, which reflects the portion of its isotopes in nature. That's why the mass number is not a whole number. It can be close to an integer if one particular isotope predominates in the native mixture, but that's just by chance. A handy extra feature is a list of the isotope ratios.
Because there are so many properties for each element, any one table must make the choice of which to pack into the boxes. That's why scientists keep several editions, collected from various sources. Two particular versions I have to hand pack 9 and 13 parms in each box. They are crammed in with real tiny print requiring a lamp and magnifier to discern. You can get comprehensive periodic tables as a computer program. You customize the display to give on the main screen only the parms you need. The others are placed into pages or layers to flip thru.
Note the gap between calcium and gallium. The element number jumps from 20 to 31. The gap is where, omitted from the simple chart here, other elements are arrayed. These include such common ones as iron, copper, cobalt, nickel, zinc. Iron, for example, is element #26.
The modern form of the periodic table comes from a better understanding of the role electrons play in chemistry. The rows of elements, including the one with the missing elements, contain those whose outer zone of electrons have the same capacity. The first row, with hydrogen and helium, has only 2 places for electrons in the outer zone. The 2nd and 3rd row have 8 places for electrons. The 4th, and a few more not shown, has 18 places for electrons.
The columns, or groups, have those elements with the same number of valence electrons. The group for boron, aluminum, gallium has three valence electrons. That for fluorine, chlorine, bromine has 7. I numbered the groups accordingly. An exception is helium, in the 8th group. It got only 2, not 8, valence electrons. So, even from the skeleton chart here, you can extract some useful facts about the elements!
The interaction of electrons becomes pretty complex for elements in the 4th and later rows, or periods. These elements can play tricks by juggling their electrons to have various numbers of valence electrons and seats. Compounds of these elements can have many combinations of atoms, not just a single simple set.
In general, elements near the left side of the table unite best with those near the right side. Those near the middle join only weakly with the ones at the sides and most weakly among themselves.
The periodic table does NOT say much about the nucleus of the atom, like the radioactivity or decay. That's because these are not the domain of chemistry. ALL isotopes of a given element have the SAME chemical behavior. For nuclear information, you need a whole other table, a table of nuclides. This lists each isotope in its own box with its peculiar nuclear properties.
The last element in my skeleton chart is 'Kr'. You can GUESS what it is! Kryton IS a real element, one of the inert gases that does not merge with other atoms into compounds.
That's it, OK? It's back to the stars. May was, or wasn't, a month with a 'blue Moon'. One common definition of a blue Moon is the second full Moon within a month. This can happen only if there's a full Moon at the very start of the month. The next one, 29-1/2 days, later, can then occur within that same month near the end.
From New York, running on EDST, there were full Moons on May 2nd and May 31st. So May was a blue Moon month. However, if we cite the full Moon occurrences in UT, as astronomy events are usually calculated, the picture is a bit different. The first full Moon fell on May 2 and the very next on June 1st. So May was NOT a blue Moon month!
~See 2nd table to the left~
So May was a blue Moon month in some time-zones, but not in others. We must be careful to note which time-zone we're basing the satisfaction of the blue Moon definition.
The time-zone shift also affects other celestial activity. Venus reaches its maximum displacement east of the Sun on June 9 03h UT. In EDST Venus does so on June 8th 23h. You will see in assorted tables of celestial events the maximum displacement occurring on the 9th (in UT) or on the 8th (adjusted for eastern US).
The correct jargon for the displacement of Venus is 'greatest eastern elongation'. Elongation is the angular distance from the Sun to Venus along the ecliptic. Recall that Venus, in its orbit interior to that of Earth, can swing from east to west side of the Sun, but only out to a certain maximum distance. Because its orbit, and Earth's, is nearly circular, this elongation is about the same on all occasion, about 46 degrees.
The diagram of the inner solar system shows the planetary situation for June 9th (in UT!). I draw the line of sight from earth to Venus and the ray of sunlight from Sun to Venus. Note that the Earth-Venus line is tangent to the Venus orbit and perpendicular to the Sun-Venus line. This right-angle between the Sun's ray and our sight line makes the phase we see on its disc. If you hold a ball in sunlight such that the Sun shines on it directly from your right, or left, you'll see a half-Moon phase on the ball. Honest; go and try it.
Note, too that Venus, circulating counter clockwise around the Sun, is at this moment moving directly toward earth. For a short distance near the June 9th position, Venus is still moving pretty much toward Earth, with little lateral motion. Thus, on the sky, Venus seems to keep pace with the Sun for a couple weeks, staying about the same angular separation from it.
The concept of 'eastern' elongation can throw some readers. 'I see Venus in the west, not the east'. Imagine yourself on Earth in the diagram. Look at the Sun.like it's noon. Isn't west on your right and east on your left? Well, there's Venus, left of the Sun, so its elongation is to the east.
We see it in the west BECAUSE it is east of the Sun and lags behind it after sunset. Converse reasoning applies to Venus as a morning star before sunrise.
In skywatching context, the GEE, to shorten the term, means the beginning of the endgame for Venus in its present apparition. From now until mid August, Venus swooshes westward toward the Sun and races past it to move into the morning sky by mid August. Trace this out in the diagram. The arc of Venus orbit from GEE to inferior conjunction, Venus between Earth and Sun, 'IC' in the diagram, is only about 1/3 of that from superior conjunction, Venus on the far side of Sun at 'SC', and GEE. Venus spends three times as long getting from superior conjunction to GEE than from GEE to inferior conjunction.
Now is the time to inspect Venus thru your friend's telescope. Ask that he use the same magnification each time, so the angular increase in the size of Venus, and the shrinking crescent phase, can be directly compared from view to view.
You'll see that as Venus grows in diameter, from its approach to Earth as it rounds into the earthward part of its orbit, Venus also wanes in illuminated area. The two factors more or less compensate to keep Venus roughly the same brilliance all the time.
This is unlike Mars, who presents a full disc to us, with only minor defect due to phasing. Its brilliance varies widely due to changing diameter. When on the far side of the Sun, Mars is a modest star, easily overlooked. When near opposition, closet to Earth, Mars ranks among the brighter 'stars' in the sky.
Jupiter remains about the same brightness because its distance from us changes only moderately, while keeping a full disc turned to us. Saturn's brightness changes year by year due to the amount of ring surface tilted to us.
In its endgame as evening star, Venus puts on two final shows. On June 18th Venus convenes with Regulus (REH-guh-luss), Saturn, Moon, and Beehive star cluster, lining up with them in a slanted row in the west. You may need binoculars to make out the Beehive cluster in strong twilight or misty summer air.
Saturn, Moon, and Venus fit within your binocular field. The Moon may show its night side in a faint glow, but not for sure. Regulus is a bit off the upper left edge of the field, Beehive. lower right.
Following this convention, watch Venus and Saturn each clear night for the rest of June. The two planets converge, night by night, until on July 1st, they almost touch together! The two form a 'double planet' as if Venus had a moon. Or, it may be what Earth and Moon look like from Mars?
After this meet, Venus quickly slides downward, leaving Saturn to linger by itself. It, too, slides downward, more slowly, to fade away in twilight by early August. In late August Saturn shifts to the dawn sky.
The charts here show the western sky on June 18th and July 1st. Bear in mind that summer in New York an being hazy humid skies, masking the fainter stars.
Our panorama chart is plotted for June 18th at 21h EDST. We look southeast, opposite Venus and Saturn, with zenith near top center.
Alkaid (al-KAYD), quite overhead, is the end star in the Big Dipper handle. The rest of the Dipper drapes in the high northwest. You should face northwest to see it more comfortably.
In this SpaceWalk we chance to face both west and east (OK< southeast), in opposite halfs of the sky. In the west, Castor (KASS-torr) and Pollux (PO-luks) are about the last of the winter stars still in sight. In south, not well depicted on the charts here, are Spica (SPIH-ka, SEE-ka) and Denebola (deh-neh-BOH-la). They are part of the spring group of stars.
In the east, we got the newly arrving stars of summer. They start with Arcturus (ark-TURR-uss) and stretch all the way to Antares (an-TA-reez), Vega (VAY-ga), and Altair (al-TAYR).
With binoculars look for the Hercules (HERR-koo-leez) star cluster, Unlike the Beehive, this is a globular cluster, a humongous ball of about 100,000 stars over a 50 light-year diameter. The binoculars will show a round condensed fuzz ball a little smaller than the Moon. Your astronomy friend's scope may separate out the individual stars around the outer fringes.
This cluster is Messier's 13th find in the 18th century, labeled 'M13' in the chart. Walk from Arcturus through Alphecca (al-FEH-ka) and continue about the same distance beyond. You come to the two stars eta and zeta in Hercules. M13 is quite 1/3 from eta to zeta.
More or less opposite to Venus you see Jupiter. At 21h it's still a bit low, perhaps behind skyline. Wait until 22h for it to rise into higher sky. Jupiter is a little dimmer and more yellow than Venus and is the only considerable luminary in that part of the sky. After Venus sets in late night, Jupiter dominates the sky until dawn.
New York
Space Society
Chapter of the National Space Society