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 prediction.
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 and observations.
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 of Relativity.
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
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 Galaxy.
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 to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.