July night sky (Article 92)
July is the month when we are most likely to be outside late in the evening and judging by the last few very hot Julys the most comfortable place to be is sitting outside as darkness settles and read on for the best July has to offer.
The star sprinkled skies reveal the sprawling constellations of summer. High near the zenith lies the brilliant white star Vega, having moved closer to this overhead point and together with Deneb in Cygnus and Altair in Aquila, they form the Summer Triangle. The image below illustrates this relationship. The latter two constellations straddle the MilkyWay, now arcing high overhead east of the zenith, from NNE to due south. Sagittarius and Scorpius still dominate the low south, but you need a good unobstructed view and no lights, to fully appreciate this region. Higher in the south lie the faint Hercules and Ophiuchus (13th constellation of the Zodiac), while coming into view now in the east are the autumn constellations of Pegasus, Pisces and Andromeda. The four stars forming the head of Draco, the Dragon are at our zenith this month, Ursa Major and the Plough are low in the NW and Cassiopeia is low in the NE, while Auriga lies low down in the north.
On 1st July at 11 pm a waning Moon will be showing 98% of its disc illuminated. The new Moon will be visible from about mid month onwards. Jupiter, the giant gas planet will be low in the south as the brightest star-like object only getting up to about 15º above the horizon. While it is too low to see much detail on the planet itself you will easily see the 4 Gallilean moons of Io, Ganymede, Europa and Callisto with binoculars (unless they are behind or in front of Jupiter). If you have binoculars try looking at Jupiter over a few nights and make a mental note of the positions of the moons. You will be amazed at how quickly they change position. On the evenings of 24th, 25th and 26th July the Moon and Jupiter will appear very close together in the sky even though the Moon is 300,000 kms. from Earth, while Jupiter is 691,000,000 kms. away!
Venus --- if you could manage to observe this planet during the whole month of July at 8:30 pm you would see that on 1st it would have 35% of its disc illuminated when it would lie 79 million km from Earth and it would be 16º above the horizon. However, by the 31st it would have only about 9% of its disc illuminated and be 49 million km from us and at 2º above the horizon. This is because Venus orbits nearer to the Sun than Earth and during July we are seeing it come towards Earth and start to swing between us and the Sun.
Our next meeting is on July 4 at 8:00pm, in Room 206, in Mary Immaculate College. This is an open night for anyone new to astronomy or who has a telescope and does not know how to make the best use of it. Free advice and instruction, with no obligation, from experienced users. Clear skies!
30-07-2007 July night sky (Article 92)
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