TSTW 4/24/08
Friday, April 25
The Moon rises at its most southerly point along the eastern horizon this month at 12:34 a.m.
Gérard de Vaucouleurs (1918-1995), French astronomer, was born 90 years ago.
Happy 85th birthday, Francis Graham-Smith!
Saturday, April 26
Happy 75th birthday, Arno Penzias!
The Iridium 43 satellite will flare to -4 magnitude around 8:38 p.m. at azimuth 99° (E) and altitude 65°, between Denebola and the Handle of the Big Dipper, during nautical twilight.
Mars is nearest Pollux this evening; both are magnitude 1.2 and orangey.
Sunday, April 27
Jupiter is near the Moon this morning before dawn.
Canadian amateur astronomer Rolf Meier became the first person to discover a comet from Canada, 30 years ago (C/1978 H1, Comet Meier).
Cartosat 2A, Indian spy satellite, and six international nanosatellites, are scheduled to be launched at 10:53 p.m. from the Satish Dhawan Space Centre near Sriharikota, India.
Monday, April 28
Last Quarter Moon; rises 2:28 a.m.; transits 7:15 a.m.; sets 12:09 p.m.; = -20°
Eugene Shoemaker (1928-1997), American astrogeologist, was born 80 years ago.
Galaxy Evolution Explorer (GALEX), orbiting ultraviolet telescope, was launched 5 years ago (2003).
UARS (Upper Atmosphere Research Satellite, deployed from Space Shuttle Discovery in 1991) will move from NW to SSE from about 9:10 to 9:15 p.m. It reaches its highest point in the sky (69° altitude, magnitude 2.1) around 9:13 p.m., near Leo, during astronomical twilight.
Tuesday, April 29
Harold Urey (1893-1981), American cosmochemist, was born 115 years ago.
Johnson-Morgan-Kron-Cousins UBV(RI)C
Harold Lester Johnson (1921-1980)
William Wilson Morgan (1906-1994)
Gerald E. Kron (1913-)
Alan William James Cousins (1903-2001)
For the nearest stars, the change in the position of the Earth in its orbit results in a tiny shift in the position of the nearby star relative to the distant background stars. This shift is called the trigonometric parallax. You can see the effect by holding your thumb up at arms length, closing your left eye, and lining up your thumb with something across the room. Now, alternate back and forth between having your right eye open and your left eye open and you'll see the position of your thumb shift relative to an object further away. Move your thumb closer, and the shift is larger. That is the essence of trigonometric parallax.
The intrinsic brightness, the apparent brightness, and the distance to a star are related as follows:
where M = the absolute magnitude of the star
and m = the apparent magnitude of the star
and d = the distance to the star in parsecs
The absolute magnitude is the apparent magnitude the star would have if it were at a distance of 10 parsecs. Looking at it another way, the absolute magnitude is a proxy for the intrinsic brightness, and the apparent magnitude is the star's apparent brightness (as seen from Earth). The distance to the star in parsecs is just d = 1/", where " is the trigonometric parallax in arcseconds (not the number pi!).
Spectroscopic parallax is a bit of a misnomer, but here's how it works for approximating the distance to main-sequence stars that are too far away to exhibit a measurable trigonometric parallax: measure the apparent magnitude of the star, and then using its spectrum to find its position on the H-R diagram, read off its absolute magnitude. Using your measured apparent magnitude and the star's estimated absolute magnitude, you can solve for d the distance in the above equation!
Here is our final installment from Issues in the Philosophy of Cosmology by George F.R. Ellis, available on astro-ph at http://arxiv.org/pdf/astro-ph/0602280. I hope you have enjoyed reading this thought-provoking and excellent survey paper as much as I have!
10 Conclusion
The physical scale of the Universe is enormous, and the images of distant objects from which we obtain our information are extremely faint. It is remarkable that we are able to understand the Universe as well as we do. An intriguing feature is the way in which the philosophy of cosmology is to a considerable degree shaped by contingent aspects of the nature of the universe--its vast scale, leading to the existence of visual horizons, and the occurrence of extreme energies in the early universe, leading to the existence of physical horizons. Philosophical issues arising in relation to cosmology would be quite different if its physical structure were very different. Furthermore in order that philosophical analysis can engage with cosmology in depth, the detailed nature of the relation between observations and theory in cosmology is relevant.
The idea of "Laws of initial conditions of the universe" seems not to be a testable idea. Scientifically, one can only describe what occurred rather than relate it to generic principles, for such principles cannot be tested. In fact any description of boundary or initial conditions for the universe seems to be just that: a description of these conditions, rather than a testable prescription of how they must be. The 'Cosmological Principle'--the universe is necessarily spatially homogeneous and isotropic is of this kind: a description of the way the initial data turned out, rather than a fundamental reason for why this should be so. Justification of this view was based by some workers on a Copernican Principle (the assumption we do not live in a privileged place in the universe), strengthened to become a Cosmological Principle; but this is a philosophical assumption--essentially, a claim that the universe must have very special initial conditions--which may or may not be true, and does not attempt a physical explanation. This kind of argument is out of fashion at present, because we now prefer generality to speciality and physical argumentation to geometrical prescription; but it was previously strongly proposed. The tenor of philosophical argument has changed.
Nevertheless there is one kind of Law of the Universe one might propose, following McCrea: namely an "Uncertainty principle in cosmology", dual to the uncertainty principle in quantum theory. Uncertainty applies on the largest scale, as we have discussed above in some detail, and also on the smallest, where it is a profound feature of quantum theory. Its basis is very different in the two cases, on the one hand (in quantum theory) being ontological in nature, on the other (in cosmology) being epistemological in nature. Nevertheless it is a key aspect of our relation to the cosmos, so that (following McCrea) we might perhaps formalize it in order to emphasize its centrality to the relation between cosmology and philosophy:
George F R Ellis
Mathematics Department and Applied Mathematics,
University of Cape Town,
Rondebosch, Cape Town 8001, South Africa.
March 29, 2006
