July 2007 Sky from the Keeble Observatory
Contemporary physics explains the various interactions between
material objects in terms of four “fundamental forces.”
The strongest is aptly named the strong force. This is extremely
short-ranged, and holds the nuclei of atoms together, as well as
binding the more-fundamental quarks which make up the structure
of protons, neutrons, and various mesons. Weaker by a factor of
almost exactly 1/137 (the so-called fine structure constant) is
the electromagnetic force, manifested as light and responsible for
binding electrons to nuclei in atoms. This force has infinite range
– it acts between any two particles anywhere in the universe
as long as they have net electric charge. Next in strength, comes
the weak force, even shorter range than the strong force, and is
responsible for the form of radioactivity known as beta emission.
Compared to the strong force, this one is 100,000 times weaker!
Weakest by far is gravity (forty orders of magnitude smaller than
the strong force – that’s a decimal point, 39 zeroes,
then a 1). Yet, gravity truly dominates the large-scale structure
of the universe. Infinite in range, like electromagnetism, gravity
acts between any two entities which having mass. That is, gravity
acts on everything!
Aristotle attributed the property of weight to “primacy of
place.” He explained that all materials were made of a combination
of four “elements” – which he labeled as earth,
water, air, and fire. Each element had a preferred place in nature,
with earth at the center, water next, then air, lastly fire. If
displaced from its proper place, an element would try to return;
thus, objects containing mostly earth would fall to the ground.
If we release the “fire” from a piece of wood, the smoke
carries the fire upward, leaving behind the ashes (mostly earth)
which settle to the bottom of the fireplace. According to this model,
the more “earth” an object contains, the heavier it
is and the faster it should fall if dropped.
Isaac Newton developed a mathematical expression for gravity which
eliminated the notion of primacy of place, substituting the role
of mass itself. In this model, all objects fall at the same rate.
Galileo had actually tested this idea, reportedly dropping a cannon
ball and a musket ball from the Leaning Tower – both hit the
ground at virtually the same instant, contradicting the Aristotelian
Newton reportedly reasoned from comparing the acceleration needed
to keep the Moon in orbit to the acceleration of a falling object
(perhaps an apple in the family orchard). Any good surveyor could
tell that the Moon orbits at 60 times the Earth’s radius from
its center, while the apple was falling at essentially 1 radius.
The ratio of these accelerations turns out to be 1/3600, or the
inverse square of the ratio of the radii. He also reasoned that
the force between two objects should be in proportion to the product
of their masses. We now write this “Law of Universal Gravitation”
as (equation alert!)
F = -GmM/r2
The minus tells us that the force is attractive. M and m are the
masses of the two objects, and r is their separation, measured center
to center. Claiming this as a universal rule may seem an exercise
in hubris, but its accuracy has been confirmed in myriad experiments
How well confirmed is this? High school and college physics students
routinely confirm this using a device known as a Cavendish balance
to measure the force between pairs of small lead spheres. It’s
the gravitational equation used to steer space probes throughout
the solar system. We use it to “weigh” stars and galaxies,
and to discover unseen planets by their gravitational interactions
with the parent star.
However, the story is not complete. Next month we’ll discuss
the current best model for gravity, Einstein’s General Theory
Lunar phases for July: Last Quarter on the 7th, at 12:54 pm; New
Moon on the 14th, at 8:04 am; and First Quarter on the 22nd, at
2:29 am; Full Moon on the 29th, at 8:48 pm.
July is also the month for the Aquarid meteor shower, which should
peak early in the morning on the 28th. From a dark site, away from
city lights, you might be able to see up to 20 “shooting stars”
per hour. Unfortunately, the almost full moon will make all but
the brightest meteors hard to see.
Pre-dawn planet watchers this month will have to content themselves
with Mars, which begins the month about 43 degrees above the ESE
horizon at sunrise. By month’s end it will climb to nearly
60 degrees. Mercury joins Mars in the twilight, rising about an
hour before the Sun by the end of the month. If you can get to an
uncluttered view, look for Mercury about 13 degrees above the ENE
horizon as the Sun rises.
Evening planet watchers will see Venus and Saturn very close together
at the beginning of the month. Look for them separated by about
the width of your index finger held at arm’s length, 30 degrees
above the western horizon.
At mid-month, our overhead view finds the constellation Hercules
at zenith about two hours after sunset. The four brightest stars
in this asterism form a trapezoidal shape called the “keystone.”
Binoculars and a clear, haze-free night (hard to come by in central
Virginia’s summertime!) should enable you to see the globular
cluster M13, about 1/3 of the way between the westernmost stars
in the keystone. This is a tightly bound group of approximately
a million stars, orbiting our Milky Way Galaxy, located about 25,000
light years away. The cluster is old – at 10 billion years,
it is perhaps twice the age of our Sun, nearly the age of the entire
Towards the east we see the familiar “summer triangle”
with bright Vega at 65 degrees from the horizon. Below and to the
left, about 45 degrees from the eastern horizon is Deneb, in Cygnus.
To the right, about 37 degrees from the horizon is Altair. While
you have your binoculars out to see M13, explore the vicinity of
Vega. You’ll see a closely spaced binary (epsilon Lyrae) which
can itself be resolved into two closely spaced pairs with a modest
telescope. Between the two moderately bright stars on the other
side of the constellation, you may be able to find the Ring Nebula
(M57 – the M stands for Messier, who compiled a catalog of
objects that he kept confusing with comets).
At this orientation, Cygnus truly resembles a flying swan, with
Deneb marking the tail, and Albireo its head. Albireo is a binary,
which a small telescope will reveal as having strikingly different
colors – one is a bright orange, the other is blue. It also
marks the plane of the Milky Way, and lies in the direction towards
which the Sun moves as it orbits the center of the Galaxy.
South of zenith we see Jupiter close to the bright red star Antares
(“rival of Mars”) in Scorpio. High to the northwest,
we see the familiar “big dipper” of Ursa Major. The
“handle” is almost vertical from the horizon, above
the “bowl” of the dipper. Binoculars may help you pick
out the spiral galaxy M51, about three degrees below and to the
left of the highest star in the handle.
For your own monthly star chart, you can direct your web browser
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