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Jupiter the ‘star’ this month

April provides us with longer nights to enjoy the night sky, but colder weather is starting to return. The bright stars of summer are starting to move lower in the west after sunset, with the stars of autumn returning in the east.

Bright stars visible this month include blue-white Rigel low in the west, similarly coloured Achenar, Canopus and Procyon higher in the west and white Sirius higher up. The planet Jupiter dominates the east, with blue-white Spica nearby. Yellow Alpha Centauri lies in the south east, near the Southern Cross.

The Milky Way stretches overhead. Although at its best in the winter, it still provides an impressive sight in our dark rural skies.

While the weather gets colder, the autumn provides backyard astronomers opportunities to see distant galaxies, which shall be described more next month.

Chart (9pm on the 15th April) produced using the Stellarium software package

The Giant Planet Jupiter

The highlight for April is the giant planet Jupiter, which shines as an unmistakable bright yellowish ‘star’ towards the north east after sunset. Jupiter was at its best for the year last month, when it rose at sunset, and hence was opposite the Sun in the sky, at its closest point to Earth and brightest for the year. This point happens every year and it called Jupiter’s ‘opposition’.

The giant planet will continue to be visible in our evening skies for a few more months to come. April gives us a good opportunity to see it well.

All about Jupiter

Jupiter is the largest of the Solar System’s eight major planets. It’s width is around 11 times that of Earth, so big that more than 1000 Earths could fit inside. In fact Jupiter is so collosal that all the other planets of the Solar System could be fit inside it with room to spare.

Luckily for us it lies more than five times the distance to the Sun away from Earth. If it was by chance a whole lot closer, then Jupiter’s immense gravity would undoubtedly have affected the Earth over its long history, perhaps interfering with the evolution of life. Although we’d get a great view, we would likely not be here to see it.

Jupiter is a ‘Gas Giant’ planet, so called because it is made of gas, namely Hydrogen and Helium. If a spaceprobe was to visit Jupiter it would descend into the brightly coloured cloudtops that we can see through our backyard telescopes. The probe would disappeare from our view and continue to fall through the clouds, encountering ever increasing pressure until it was eventually crushed by the pressures deep within the planet’s bulk.

It would never reach a solid surface, as Jupiter does not have one, the poor probe would simply fall forever.

Jupiter carries with it a large retinue of moons, among which are four very big ones. These four, called Io, Europa, Ganymede and Callisto are easily visible through small backyard telescopes. They can actually be seen through a pair of binoculars if held steadily.

These moons continually move around Jupiter and sometimes pass in front of the planet, casting a shadow onto the cloud tops beyond. These ‘shadow transits’ are quite easy to see, particularly those caused by Ganymede, a moon which is bigger than the planet Mercury.

Have a look at Jupiter through a pair of binoculars and you will see the moons. If you have a telescope you will also see the planet’s main cloud bands and atmospheric storms, as well as the shadows of the moons when the transits happen.

Jupiter rotates once in just under ten hours, looking at it over an hour can reveal how the features have moved. The cloud patterns and features of Jupiter change all the time. By keeping an eye on it you will be helping astronomers keep track of the weather on another world.

In April our Moon is at last quarter (half) phase on the 1st, new on the 8th, at first quarter (half) phase on the 15th, and full on the 23rd. Have a look when it is full, the dark patches are thousands of millions of years old.

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