Comet C/2002 V1 (NEAT) - update 27/1/03

The comet's path for the coming 20 days. The 6 degree circle gives the average field for a 7x50 binocular. Click the image for an enlargement.

The comet is now an easy binocular object with a small tail and should become visible to the naked eye within the next week. (The Moon starts to become a problem after February 4.)

Some predictions are suggesting the comet could become bright enough to be seen by the unaided eye in strong twilight.

Jupiter at opposition - February 02, 2003

Jupiter is at its closest to the Earth for the year on February 1st 2003, opposition occurring the following day at around 09 hrs. For Jupiter opposition and time of closest approach are never far apart, unlike Mars. This is because the eccentricity of Jupiter's orbit (0.0485) is less than is the case with Mars's orbit (0.0934).

In 2001 Mars was at opposition on the 13 June whereas closest approach to Earth took place on the 22 June. In 2003, on the other hand, Mars will be at its closest to Earth on 27 August at 10 hrs, opposition occurring the following day at 18 hrs. Mercury has the most eccentric orbit (0.206) and Venus the least (0.007).

Dia. 2 Track of Jupiter from Feb 02 to May 30 2003. The part-circle indicates the position of "Preasepe" open cluster (M44) in Cancer. Click for enlargement.

The general progression of the superior planets (those with orbits outside the orbit of the Earth) is from west to east against the background of the stars. However, since the apparent movement of any planet is a compound of two movements - that of the Earth and the planet, this forward motion is interrupted by a period during which the planet retrogrades - moves east to west against the stars (remember, these notes are for the northern hemisphere).

Jupiter is currently in retrograde motion and will remain so until April 1st - see dia. 2. [The diagram shows stars to magnitude 9 - visible in a 30 mm binocular - and labels to magnitude 7, the decimal point having been omitted; thus, read the star d (Cancri) as magnitude 3.93. In the interests of clarity the designation of all other stars has been omitted.]

Diagram 1 shows the sky looking south at 23 hrs (11 pm) on February 2nd. Both Saturn and Jupiter make a startling addition to the splendour of the winter sky.

Diagram 1: click for larger view

Jupiter is in many ways the most rewarding planet to the observer with a telescope of moderate power. It always presents a sizable disc and even with powers as low as x30 it is possible to see some surface detail (usually two bands parallel to the Jovian equator) and to follow the movement of the four Galilean satellites, their eclipses and transits. Jupiter rotates on its axis in 9.84 hrs. The resulting equatorial bulge is readily visible in small telescopes.

The four Galilaen satellites are amongst the largest satellites in the solar system. Gannymede, the largest (dia. 5262 km), is rivaled by Saturn's Titan (5150 km). Of the remainder, Europa is the smallest (3138 km). At this coming opposition Jupiter will have an apparent angular equatorial diameter of 45.5 arc seconds. This means that Gannymede will appear 1.68 arc seconds, quite within the resolving power of a good 100 mm refractor.

To see the "disc" of Gannymede will require a power of around x180. It is best to make the observation in strong twilight. Jupiter is comparatively easy to pick up in daylight with a telescope and with the "go-to" facility offered by many telescopes these days the fete may be accomplished without effort. For those with more modest equipment we shall be discussing observations in daylight in a forthcoming issue of Sky Notes. All four satellites are generally brighter than magnitude 6 and would be visible to the naked eye were it not for the close proximity of the much brighter Jupiter.

In July 1994 a number of fragments from Comet Shoemaker-Levy impacted with Jupiter. The circumstances had been predicted but the visible effects surprised most astronomers. Between July 16 and July 22 the planet was subjected to a "bombardment" from 20 fragments. The disturbance caused to Jupiter's deep and complex atmosphere could be observed over many days using telescopes as small as 75 mm aperture. Some of these were visible in my Wray 85 mm equatorial refractor working at x110.

To have had the opportunity of observing such an event in one's lifetime seems in retrospect very remarkable. It serves to emphasize the mix that astronomy offers from highly predictable events - eclipses, visibility of planets etc - to the totally unpredictable. Thus, when working at the London Planetarium in 1960 I noted the rare planetary line up of the naked eye planets for April 2002; but I was not to know then that there would be two naked eye comets in the same sky!

Note: On 2nd February 2003 Jupiter rises NEE at 16 10, 22 minutes before sunset.

Star "Magnitudes"

On a perfectly clear, dark moonless night free from light pollution, the impression to the untrained observer is of an inordinate number of stars. In fact at any one time there are unlikely to be more than 2500 stars above the horizon visible to the average human eye.

What is immediately apparent is the fact that the stars are of different brightness. A deeper inspection will show that the number of truly bright stars is comparatively low. In the familiar figure of the Plough, the star forming the top left-hand corner of the "bowl" is considerably fainter than the rest.

The Plough hangs over the NW in the early evenings of autumn. If one follows the curve of the handle backwards, right to left, or southwards, a brilliant star of a slight orange tinge immediately arrests the attention. This is Arcturus, the brightest star north of the celestial equator and one of the three bright stars of the northern hemisphere; the others being, Vega (see previous notes) and Capella. These three stars are all of about the same brightness and are considerably brighter than anything in the Plough.

In about 150 A.D., long before the invention of the telescope, Ptolemy classified the twenty brightest stars as of the first magnitude; those stars just visible to the eye were said to be of the sixth magnitude. [The word "magnitude" is not to be confused with its more general usage which implies physical size.]

It was not until the nineteenth century that due attention started to be paid to star magnitudes. In 1827 Sir John Herschel established the relationship between perceived brightness and magnitude by noting that a star representative of the first magnitude was approximately one hundred times as bright as a star of the sixth magnitude. The system has been much refined since then.

It can be said that stars differing from each other by one whole magnitude are perceived to be in a ratio of brightness of 1:2.5. Thus, a star of the first magnitude is 2.5 times as bright as one of the second magnitude, and one of the second magnitude is 2.5 times as bright as one of the third, and so on; but a first magnitude star is 6.3 times as bright as one of the third.

On this scale it will be seen that the brighter the star the smaller the number representing its magnitude. For the brighter stars negative values are applied. Sirius, the brightest star in the entire sky, is of magnitude - 1.46; Arcturus is -0.04.

But the perceived brightness of an object is a function of the receiver. The human eye shows greatest sensitivity for light in the yellow/orange part of the spectrum. There are even differences between individual observers and so it is necessary to be precise when recording magnitudes. [Comparing the images of stars taken by photographic means will yield results depending upon the range of sensitivity of the film or emulsion used.]

Whenever magnitudes are mentioned in these notes "visual magnitude", as opposed to, say, "photographic magnitude", is to be understood.

Magnitudes may be determined to an accuracy of 1/100th of a magnitude using photographic or photoelectric methods. Experienced observers may estimate to the order of 1/10th of magnitude or even better.

Magnitudes may be assigned to the planets as well as the Sun and the Moon (and artificial satellites for that matter). The full Moon can be about -12.5 whereas as a slender 24 hour-old crescent it is a mere -5.3, or less than twice as bright as Venus at maximum. Venus itself ranges between magnitude -3.8 and -4.7. Jupiter at closest opposition can reach -2.9; Mars, also, at maximum - see opposition 2003. Saturn with the rings fully open can reach magnitude -0.5, as it did in 2002 and will again in 2003. Uranus, not strictly regarded as a naked eye planet, attains 5.7 magnitude at opposition next August. Pluto, at magnitude 14, requires a telescope of at least 8 inches aperture to be seen.

The magnitudes of stars are recorded in photometric catalogues. The magnitudes for bright stars in these notes are derived from the Bright Stars Catalogue 5th Revised Edition, Yale Observatory 1991, unless stated otherwise. There are many stars showing characteristic fluctuations in their light - the variable stars. There are some dramatic examples in the group known as irregular variables. The most notable of these is Mira in Cetus.

At maximum Mira may be of the second magnitude, at minimum no brighter than magnitude 10. Betelgeuse is another irregular variable. Usually it is below Rigel in brightness but it may at maximum rival the brighter, blue star. Two further important class of variables are the "Cepheids" (named after the prototype, d Cephei), and the Eclipsing Binaries or "Algol" type (named after b Persei or "Algol"). Such stars show regular light curves.

Star classification and magnitudes

As a general rule the brightest star in a constellation is marked by the letter 'a', the next brightest by 'b'', and so on through the Greek alphabet followed by the genitive of the constellation name. There are many exceptions to this as will be explained in a later feature in these notes.

The fainter stars use other means of classification. Flamsteed (First Astronomer Royal) designated stars within a given constellation numerically in order of Right Ascension, for example.

Variable stars have been mentioned, but small differences in tabulated magnitudes sometimes appear for those stars not adequately studied and understood as variable. This may account for some of the discrepancies given in tables to be found in various publications. But inconsistencies of another sort sometimes creep in.

Confusion most often arises in the case of close doubles - stars too close together to be seen as separate stars to the naked eye*. A good example is Castor, (a Geminorum). Castor comprises two stars of magnitudes 1.9 and 2.9 about 5" apart. [The system is a true binary so the distance between the two components alters with time.] The combined magnitude for Castor is 1.59, but some tables of stars may give the magnitude of the primary or secondary without actually stating as much. In these notes when the magnitude of a double stars is given, the figure may be taken to represent the combined magnitude of the pair.

It should be noted that the combined magnitude is not obtained simply by algebraically adding the two magnitudes. Those interested in studying the subject further should consult a reference book such as Nortons Star Atlas & Reference Hanbook (published latterly as NORTON'S 2000.0, edited by Ian Ridpath).

* A feature on double stars will appear later in 2003.

J V
December 24, 2002