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Visible Solar Activity - October 2003
There have been a number of interesting sunspot formations this month. At the time of writing (23/10/03 13 00h GMT) there are two large groups visible. The largest is close to the Sun's centre and is tracking close to the solar equator. (This group is readily visible to the naked eye using appropriate screening/filtering techniques.) The second group close to the eastern limb occupies a slightly lower southern solar latitude. Auroral activity has been sporadic over the past week. With the current level of solar activity there could well be some interesting auroral displays over the coming days. Saturn
2003/2004 Observers in Orkney and Shetland who have followed the recent opposition of Mars through a telescope will no doubt have been disappointed with the quality of the image. At best one could make out the broader dark features together with the south polar ice cap; any finer detail has been smudged out by poor seeing, almost entirely as a result of the planet's low altitude. The situation with Mars this year will have emphasized the disadvantage of observing planets at altitudes as low as 15°. The atmosphere is seldom steady enough to allow high powers with an object at an altitude of less than 20°. This places observers in high latitudes at a distinct disadvantage where the planets are concerned. The altitude of the celestial equator at a meridian is a function of the latitude of the observer. At the equator it is 90°, at the poles it is zero - the horizon and the celestial equator are sensibly the same. From Orkney this angle is close to 30°.8. It follows, therefore, that a planet with a declination of less than -10° cannot rise at any time during the day or night above 20° altitude The position of summer solstice lies on the ecliptic at a declination of 23°.44. This, incidentally, is the angle (called the obliquity) the equator makes with the plane of the Earth's orbit. Since the planets and the Moon, in the main, follow the path of the ecliptic it follows that the maximum altitude a planet can have when on the meridian (in the northern hemisphere) will occur when it is in the constellation Gemini. The most advantaged position for a planet will be when it comes to opposition in high declination. When this occurs in Gemini the altitude may be as high as 55°, as will be the case for Saturn this year. Saturn offers the northern observer a much better deal than does Mars. The appearance of the ring system goes through a cycle from full to full (or closed to closed) every 14.7 years. The rings in 2003/4 will be at their most advantageous. In 14.7 years time they will again be fully "open" but the planet will then be in the constellation Scorpius and even lower in northern skies than was Mars this year. These southern oppositions, however, take place with the planet at a greater distance from Earth and therefore its apparent size and overall brightness is slightly less than for northern oppositions. Thus at its 2018 opposition Saturn's distance will be approximately 10% greater than its distance at opposition on December 31 this year. As a result the apparent diameter will be proportionately less and the resulting apparent magnitude will be 0.0 compared to -0.5 in 2003.
For the next twelve months Saturn will remain wholly within the constellation Gemini (dia. 1). It will be seen from the diagram that Saturn is in retrograde motion at the time of this article and will continue so until March 08 2004. From then it will move forward but will still be within Gemini by October next year, reaching another stationary point at November 09. Throughout this period, therefore, the planet will remain in high declination, above 20°. Every opportunity should be taken to observe the planets telescopically when they are in high declination. Viewing Saturn
Good seeing and transparency are essential for viewing Saturn. The planet's surface brightness (this includes the rings) is generally too low to allow fast exposures to be used even with digital cameras. The stunning photographs frequently to be seen in magazines today, are usually the result of processing based on composites made by "stacking" perhaps dozens of images and captured using telescopes of at least 250mm aperture. The Cassini division may be observed (applicable when the planet is favourably placed with the rings "open") using good refractors of around 85 mm aperture at x100. The photograph in fig. 1, obtained from a single exposure, indicates rather vaguely the Cassini division and the shadow of the rings on the ball of the planet itself. The human eye, using the same instrument, may be capable of seeing finer detail in moments of superlative seeing (lasting but a few seconds or less). A more extended article featuring the use of digital cameras for astro photography will appear shortly. The Moon The Harvest and Hunter's Moons were discussed in the previous article (see archive). Mention has been made (above) regarding the Sun's altitude at the summer solstice. The Moon's motion has also been discussed in very general terms. Both the eccentricity and inclination of our natural satellite's orbit fluctuates significantly with time. One aspect of this may be observed on the 17th October when, at around 04 00 GMT, the Moon will reach its maximum declination (27°.1, or about 3°.5 higher than the Sun at summer solstice) for this lunar cycle.
This means that on the evening of the 17th the Moon will rise at 19 52 GMT, some 18° north of NE and will remain above the horizon for 20 hours, setting at about 15 54 GMT the following day. At 14 hrs GMT the same day the Moon will be 5° north of Saturn. Observers with small telescopes could take this opportunity, using the Moon as a reference, to try to locate Saturn in daylight. Saturn is the most difficult of the naked eye planets to find in the daytime sky. However, it should be possible to see the planet in apertures as low as 60mm. JV 14/10/03 |
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