August 2007 Sky from the Keeble Observatory
Last month we looked at Newton’s “Law of Universal Gravitation,” and promised to write this month about our best current formulation of gravity, Einstein’s General Relativity.
First, however, I’d like to inject a note about the twin rovers currently on Mars. Both of these robotic explorers are well past their designed lifetime of 90 days – they’ve been actively returning data for three years. They are currently in some stress because of a huge dust storm affecting much of the southern hemisphere of the red planet. Both rely on solar energy, and the dust storm has reduced the intensity of sunlight reaching the probes to dangerous levels. They typically require about 200 watt hours of energy per day for minimal survival, but are receiving less than 150. Controllers are running the probes at minimal power, and have not moved them for several weeks in order to conserve energy for the overnight heaters required to keep the electronics healthy. More about this next month.
Newtonian gravitation has been successfully used to calculate the orbits of planets and moons, and is the computational basis for navigating between planets in our solar system. We can also apply it to determine the mass and orbits of stars and galaxies. In spite of the seeming hubris of calling it universal, it works! However, if we push Newtonian theory to extremes of large mass, it begins to break down. For example, if we observe the orbit of the planet Mercury (closest to the Sun) and compare it with that calculated from Newtonian gravitation, it’s not quite right. Mercury misses its calculated position by about half the diameter of the planet on each orbit.
Einstein began his exploration of gravity by asking simple questions. What would be the difference between standing on the Earth’s surface, where objects fall at a constant acceleration, and riding in an elevator way out in space which was accelerating “upward” at the same rate? He reasoned that there would be no observable difference, and hence decided that these two scenarios were exactly the same. From this “principal of equivalence” he was able to construct his General Theory of Relativity – essentially a new formulation for gravity itself.
General relativity tells us that the presence of mass warps the geometry of space and time, causing objects to move on curved paths rather than in straight lines. What Newton attributed to a “force” Einstein attributes to geometry. The mathematics is more sophisticated than Newton’s, but on the scale of the solar system and beyond, Relativity reduces to Newtonian gravitation. However, on the scale of the orbit of Mercury orbiting the mass of the Sun, Relativity gives us the correct orbit.
One consequence of this picture is that even light will follow curved paths due to the distortion of space-time. The deflection of starlight passing the Sun was measured in 1919 by British astronomer Sir Arthur Eddington, and was found to be as predicted by Einstein. It was this successful measurement that transformed Einstein from a rather obscure scientist into a cultural icon.
Relativity predicts exotic behavior under extreme conditions. For example, if enough mass is concentrated in a small enough volume, the curvature of space-time makes it impossible for light to escape. This is the so called “black hole” – once believed to be simply a theoretical curiosity, now thought to occupy the central regions of most galaxies. If a mass is rotating, it will “drag” the local space-time distortion around with it. This, too, has been observationally confirmed, most recently by a satellite called Gravity Probe B, but also in studies of pulsars in binary systems.
Lunar phases for August: Last Quarter on the 5th, at 5:20 pm; New Moon on the 12th, at 7:03 pm; First Quarter on the 20th, at 7:54 pm; Full Moon on the 28th, at 6:35 am.
Full Moon will be accompanied by a total lunar eclipse – the Moon will be entirely within Earth’s shadow. The penumbral eclipse begins at 3:52 am, but you’ll not likely notice this piece. The Moon slips into the central, darkest part of Earth’s shadow beginning at 4:51, with totality beginning at 5:52. Though totality lasts ninety minutes, you’ll not see it from here – moonset and sunrise occur at 6:35. For the best view, take your vacation to Hawaii, where the Moon will be high in the sky for the entire show.Early in the month you will see Mercury in the pre-dawn skies, rising to the east-northeast about an hour before the Sun. By midmonth it disappears into the solar glare, reappearing by the end of August emerging low on the western horizon at sunset. Mars rises about 1:00 am early this month, and finds itself high to the south-southeast at sunrise. It will clear the horizon a bit earlier each day, moving high to the south at sunrise by month’s end. Venus replaces Mercury by the last week of the month, with Saturn also coming out of the Sun’s glare by the end of August.
Saturn begins the month with Venus, very low to the west at sunset, but moves behind the Sun at midmonth. Venus actually moves in front of the Sun, but disappears in its glare, anyway. Jupiter is near Antares, in the constellation Scorpio, all month, bright to the south at sunset. Antares will be red – don’t confuse it with Mars – while Jupiter will be bright and white.
Looking overhead at mid-month, about two hours after sunset, you will see the bright star Vega in Lyra nearly at zenith. The “summer triangle,” with Vega, Deneb, and Altair will be among the first bright stars to emerge from twilight. Deneb is to the east of Vega, in the constellation Cygnus. Altair is to the southeast, in Aquila. In the 20 July issue of the journal Science, researchers report that they have reconstructed an image of Altair using multiple telescopes as an “interferometer.” As a general rule, stars are so far away that the light we receive comes effectively from a single point. The star is rapidly rotating, an oblate spheroid rather than a nearly perfect sphere like the Sun.
At the other end of Cygnus from Deneb we find the binary Albireo, resolved with binoculars, impressive with two distinctly colored stars in a modest telescope. Cygnus lies parallel to the Milky Way, which runs north-northeast to south-southwest. Below Cygnus, towards the eastern horizon we are looking out of the plane of our Galaxy. This relatively open patch of sky is marked by the broad “Great Square” of Pegasus. To the west of zenith, a smaller irregular rectangle marks the constellation Hercules. Closer to the horizon, the bright star Arcturus is found in Bootes.
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.
Copyright 2007George Spagna