The Night Sky this Month

April 2011 by Russell Eberst

With the Sun now north of the Equator, the night are steadily growing shorter. The stars of Spring hold sway in the night sky, with groups like Leo, Ursa Major, and stars such as Arcturus and Spica, being well placed for observation. Spica is the brightest star in Virgo, the constellation of the virgin. It is often represent as the ear of wheat being held by the maiden. Spica has an apparent magnitude close to +1.0, so is a good example of a first magnitude star. It is usually the brightest object in a wide area around it, but at present it is outshone by the planet Saturn just a few degrees away to the north-west.

The planet Saturn reaches opposition on April 4, and will be visible all night. Its ring system, which is the most identifiable characteristic of this gas giant planet, is gradually opening out from the edge-on orientation that it reached in 2009. Now it has achieved a tilt of almost 10° to our line of sight, meaning that features such as the Cassini division in the rings are detectable in amateur telescopes. Also visible in such telescopes are the main Saturnian moons such as Titan. During the month, Saturn is retrograding, steadily moving towards the star gamma Virginis (also named Porrima or Arich). The Full Moon passes 8° to the south of Saturn oon the night of April 16-17. The Moon is one day before its closest to the Earth (perigee), so will appear larger than usual, enhancing the 'Moon Illusion' especially when close to the horizon at rising and setting.

Saturn is about 800 million miles distant, or about 3600 times further away than the Moon. The other planets are all poorly placed with Venus very low in the eastern dawn sky, and Mercury, Mars and Jupiter all lost in the glare of the Sun. There are several conjunctions among these three planets (along with Uranus) that occur during April, but the overwhelming glow of the Sun render them all invisible for observers over much of the Earth.

There are no bright comets forecast for April, and the main meteor shower is not one of the most prolific showers of the year. It is known as the Lyrid shower, since the meteors appear to radiate from a point near the bright star Vega, in the constellation of Lyra close to its border with the neighbouring Hercules. Although up to 20 meteors per hour are predicted, the presence of the Full Moon around the time of the shower maximum, on April 20-21, will deter all but the most dedicated of meteor observers.

April 2011 sees a whole host of important anniversaries. The most important of these is the 50th anniversary of human spaceflight. At the height of the 'Space Race' between Soviet Union and the U.S.A., the aim was to be first to get a man into orbit around the earth. This was achieved on April 12, 1961, when Yuri Gagarin made a single circumnavigation of our planet. His 89 minute orbit, that ranged from 169 to 315km above the earth, marked an achievement that confirmed the Russian lead in taking humans into a new, unknown realm.

There have been well over 500 people in orbit over the past 50 years, and numbers are likely to increase steadily despite the fact that the U.S. shuttle programme is coming to an end this year. The shuttle provides our second major anniversary, since it is 30 years since the first shuttle launch. The space shuttle Columbia was launched on a two-day mission on April 12, 1981. Onboard were astronauts Robert Crippen and John Young (who had been to the Moon on the Apollo 16 mission in April 1972). Columbia successfully completed 27 missions, but when returning to Earth on its 28th mission, it broke up on reentry on February 1, 2003, killing all seven astronauts on board.

40 years ago, in 1971, the Soviet Union also suffered the death of returning cosmonauts. Volkov, Patsayev and Dobrovolsky who had spent 23 days aboard the Soviet space station Salyut 1 were killed when their spacecraft, Soyuz 11, depressurised.

A happier anniversary from 1971 was the launch of Prospero, the first and only U.K. satellite to be carried into orbit on a U.K. built rocket. It was launched from Woomera in Australia, and is still orbiting some 40 years later at heights between 529 and 1328 km. The last stage of the Black Knight carrier rocket is also still in orbit at heights similar to its payload.

A 21st anniversary that occurs in April is that of the launch of the Hubble Space Telescope which took place on April 24, 1990, aboard the shuttle Discovery which has now been retired from service. It initially orbited at a height of 616 km but this figure has dropped over the years to 563 km due to atmospheric drag. The 2.4 metre telescope displayed optical imperfections at first, but these were corrected subsequently. It has since returned many thousands of images from many areas of astronomical research, and capturing spectacular events such as the impact of comet Shoemaker-Levy 9 with Jupiter in July 1994.The telescope has now been serviced for the last time, and plans for safe disposal must be made before it decays naturally in the atmosphere during the second half of this decade.

On this side of the Atlantic, the Visitor Centre at the Royal Observatory, Edinburgh, was opened in 1981. Now, 30 years later, the Visitor Centre, continues to thrive. The Centre occupies much of the East tower at the Observatory. This landmark dominates the site on Blackford Hill with its huge turret newly recovered in copper. The Observatory now provides facilities for the public to partake in night sky viewing, and to learn about the methods and results that exist in modern astronomy.

The centre opened on April 24, but the weather decided that we had not had our complete winter supply of snow, and provided an appreciable fall of the white stuff. The centre was declared open by Patrick (now Sir Patrick) Moore. The centre attracted a steady stream of visitors, numbering almost 10000 per year. The time and effort put in by staff to ensure the success of the venture was recognised by the award of First Prize in the" Museum of the Year" competition for 1982.

On 1985 November 16, with the approach of Halley's comet many hundreds of the public attended a six-hour long observing session. People were willing to queue for up to 90 minutes to get a few seconds glimpse of this once-in-a-lifetime visitor. The Centre has hosted several media events, especially when a sky spectacle was predicted. These include the passage of comets such as Hale-Bopp in 1997, the transit of Venus across the Sun's disc in June 2004, and several solar and lunar eclipses.

Further displays of astronomical interest were introduced including the Starlab inflatable mobile planetarium, the exhibition of meteorites, and demonstrations showing the production of a mini-comet. Whereas the Space shuttle and the Hubble Space telescope are approaching the end of their active life, the Visitor Centre seems likely to continue for much longer thanks to the efforts of the dedicated staff who look forward to the next quarter of a million visitors.

Having looked back to a number of anniversaries, we should look forward some 50 years, to the year 2061, when Halley's comet will be making its next approach to the Earth and the inner solar system.


Moon phases:

 

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