Newspaper
Bookshop
Online Business
Advertising
Services
The Company
Contact Us
Search Site
Site Statistics
|
Understanding
Sky Charts Fireball of March 2 I should like to thank all those who contacted me via the website. I am in the process of analysing the observations and more will appear in these notes shortly. It was good to have contact with some over the telephone and to have their comments about Skynotes. I shall endeavour to take on board suggestions for improving the site. I think it is evident that a number of people would like more detail on locating things in the sky in relation to their horizon. This is notoriously difficult to portray where one is trying to put three dimension into two, to say nothing of the problems of light pollution etc. There will be a feature on binoculars mostly for those starting off . This will include some tips on finding your way around the sky. Observationally there are three major events on the calendar to be covered in some detail: May 7, Transit of Mercury; May 31, Annular Eclipse of Sun; August 28, Opposition of Mars. Jupiter satellite phenomenon
Jupiter is still retrograding towards the open cluster M 44 (Beehive Cluster). This cluster is visible to the naked eye as a fuzzy patch a little over a degree of arc in diameter (twice the apparent diameter of a full Moon). Dia. 1 shows Jupiter in relation to the cluster on April 03 at 20h 00m; the circle indicates a field of 6 degrees typical for a 8x40 binocular. Stars are shown to 9.5 magnitude. The planet is "stationary" on April 3/4, after which it resumes its forward motion towards Leo. Those who have been following the planet (despite the weather some sort of observing has been possible!) cannot fail to have noticed a number of close approaches between the satellites - frequently referred to as "mutual events". These continue throughout the following weeks. Three interesting examples occur as follows:
March 25/26: Between 20h 44m 51s and 21h 51m 35s Europa occults Ganymede (the two satellites will be in line as seen from the Earth). Some time later in the early hours of the 26th (00h 06m 55 to 00h 12m 14s) Ganymede is eclipsed by Europa. In this instance Ganymede passes into the shadow of Europa. Dia. 2 shows the distribution of the satellites just prior to the eclipse. The separation between the two satellites of about 60 arc seconds at the time this event takes place is of course due to the fact that the planet and its system is now well past opposition. The shadows therefore no longer fall behind the planets and its satellites as seen from Earth but rather appear to the left. What the observers sees, then, is the disappearance of Ganymede without any apparent reason for it! April 1/2. A similar event takes place involving the same pair of satellites. The occultation is between 23h 53m 38m and 23h 59m 06s. The shadow eclipse takes place on April 02 between 03h 31m 12s and 03h 33m 14s. March 28. In the early hours there are occultation/eclipse phenomena involving Io and Europa as follows: Occultation 01h 55m 44s and 01h 59m 07s; eclipse 03h 31m 12s and 03h 33m 14s. For the events mentioned, Jupiter will still be well above the horizon. However, with the advancing season, and Jupiter starting to lose declination, opportunities for observing the planet become severely compromised. Auroral archive There has been a delay in setting up the archive. I should be pleased to hear from anybody prepared to contribute. Mercury at eastern elongation - April 2003 This section should be read in conjunction with the accompanying article "Transit of Mercury". Observers will recall that in April 2002 we had a spectacular alignment of all the naked eye planets. Those who missed the opportunity to see Mercury then should have a good opportunity to observe the planet during this April.
Mercury commences the month with an early evening elongation of 11º when it sets 1h 11m after the Sun. At a magnitude of -1.4 (about as bright as Sirius), it should be visible very low down a little north of west at around 19h 40m GMT. The following evening the young crescent Moon will be 4.5º lower still towards the west point of the horizon. (Map2). Greatest elongation (20º) takes place on the16th when the planet sets 2h 26m after the Sun. The diagram also shows Mercury's position from the 2nd to the 17th at 3 day intervals. Throughout this time Mercury fades so that by the 16th it will be 0.1 magnitude, or about as bright as Arcturus. From the 17th, Mercury continues to fade as it closes towards the Sun. This time inferior conjunction also sees a transit of the planet across the Sun's disc.
TRANSIT OF MERCURY - May 7, 2003
The two planets Mercury and Venus move about the Sun in orbits within the orbit of the Earth (fig. 1). This means that both planets can pass between the Earth and the Sun, and when such a passage occurs with the Earth, Sun and planet in a straight line, the planet will be seen from the Earth to pass across the face of the Sun. Such an event is called a Transit of Mercury or Venus. The situation in which a planet, the Sun or the Moon come to occupy a position in line with the Earth is termed a conjunction. The interval of time required for a planet to make one complete circuit of the Sun as seen from the Sun is called the Sidereal Period. The Sidereal Period for the Earth is 365.256 days, for Venus 224.701 days, and for Mercury 87.969 days. The interval of time between two successive conjunctions with the Sun (as seen from the Earth) is known as the planet's Synodic Period. For Venus the Synodic Period is 583.92 days, and for Mercury 115.88 days. When the planet is at conjunction on the nearside to the Earth the conjunction is called "inferior"; when on the far side of the Sun, it is called "superior". Were the planets to revolve about the Sun in exactly the same plane (imagine the Sun and planets to remain on a flat surface), then at every inferior conjunction Venus and Mercury would pass across the Sun's face as seen from the Earth. However, because no two planetary orbits lie in exactly the same plane it is only occasionally that these planets pass in front of the Sun when viewed from Earth. In the case of Mercury such events happen at intervals of either 7 or 13 years. With Venus the transits appear in pairs separated by approximately 8 years. However, a recurrence of the event at the same node can only occur after 235 or 243 years. It will be seen, therefore, that Transits of Mercury are relatively frequent compared to transits of Venus. (The next transit of Venus, as it happens, occurs next year, 2004).
Mercury At a remote superior conjunction the planet may present a diameter of around 4.6 arc seconds, whilst at a close inferior conjunction the disc can appear as much as 12.2 arc seconds.
In the telescope Mercury exhibits the full range of phases (like the Moon), being "full" at superior conjunction and "new" at inferior conjunction (fig. 2). The combination of distance and phase gives rise to an even wider range in the planet's apparent brightness (magnitude) as seen from Earth than is generally the case for any other planet. For example, when in transit against the Sun's disc no light from the Sun can reach the surface of Mercury turned to Earth so that its brightness from here is non existent. Separated from the Sun in the sky by an elongation of a little under 2 arc degrees, and with a phase of only 1%, the magnitude may be of the order of 6, resembling a star, if it could be seen in a perfectly dark sky, that is on the threshold of naked eye visibility.
[A similar set of circumstances, purely in geometrical terms, pertains to Venus: even when the planet appears close to the Sun's disc, Venus's atmosphere (Mercury has no appreciable atmosphere) will allow some light through in the direction of earth. Indeed, when the phase is of the order of 2%, a thin ring of light may be seen to encircle the disc (fig. 3).] At the other extreme, near superior conjunction, Mercury can appear as bright as the planet Jupiter at mean opposition, that is to say about twice as bright as the brightest star, Sirius. Then, of course the planet is too close to the Sun to be observed other than by an experienced observer using a telescope and taking stringent precautions against viewing the Sun itself. Mercury, because of the form of its orbit, can never be observed against a dark sky as seen from the Northern hemisphere. It appears in twilight either of an evening (eastern elongations) or in the morning (western elongations) before sunrise, and then only for brief periods of a fortnight or so. Even during a single elongation, the planet can alter in brightness from a brilliant -1.7 magnitude (brighter than Sirius) to a mere second magnitude, or about as bright as Polaris. From what has been said it might be concluded that Mercury is difficult to locate in the daytime or twilight sky. The difficulties have been overstated. When nearing superior conjunction the perceived surface brightness of Mercury may appear in the telescope to rival or even exceed Venus itself. Disregarding the modern breed of astronomical telescopes for amateurs which are now equipped with sophisticated "go to" computer technology, it is quite possible to find the planet in the daytime sky over a wide range of its orbit using quite modest equipment. However, it cannot be too strongly emphasized that any observation in daylight in the region of the Sun must be undertaken with extreme caution. Viewing the Sun direct with any optical device will almost certainly result in permanent sight impairment if not instant blinding. The Transit, May 7 The exact time at which the planet appear on the face of the Sun will vary from observing site to observing site. The times in this article are for the location of Kirkwall, Orkney, UK.
First contact will take place at 05 10 GMT*. The planet will then appear to move from left to right across the Sun's disc (see fig. 4), the transit ending at 10 31 GMT. At first contact the Sun will have an altitude of 7.5 º and 44.5º at the end of the transit so that the entire event will be visible with ease, weather permitting. The size of the disc will be very small - about 0.006 (0.6%) of the Sun's apparent diameter. In other words, if the Sun's image is projected using a telescope to produce an image 254 mm (10 inches) in diameter, Mercury will be a dot about 1.5 mm across. In order to produce a projected image of sufficient quality, a telescope of no less than 50 mm aperture should be used. A binocular suitably mounted with one lens blanked off may also serve, but the screen will have to be some way back from the eyepiece to get a disc of reasonable size. The comparatively low altitude of the Sun at the commencement of the transit will also degrade the projected image. Again it must be emphasized that this event should only be observed by projection and even then precautions must be taken to avoid anybody moving into the path of the projeced image - make sure small children are not at risk. Some forthcoming transits of Mercury:
J
V Recent photographs from the transit of Mercury
In photographs 1 & 2 Mercury is seen above and slightly to the right of the elongated sunspot.
In photograph 3, Mercury is seen on the Sun's limb. The transit ended 2 minutes later. The elongated sunspot is seen well in Hµ . Note: the uneven brightness in 2 & 3 is a result of optical effects within the telescope. JV 04/06
|
|
