The Night Sky in July 2006

Looking at the night sky with the naked eye or using a small telescope can be fascinating, so each month these notes keep you up-to-date with unfolding events.

Maps of the current night sky can be seen by clicking HERE

Phases of the Moon in July


New Moon Tuesday 25	First quarter Monday 3	Full Moon Tuesday 11	Last quarter Monday 17

Click HERE to see the phase of the Moon on each day of the month.

Monthly Highlights

With the longest day of the year (21st June) behind us it's now full steam ahead towards winter. It's a slow old process through the warmest months of the year, but as the days roll on, the nights will start to draw in. On the up side, the longer nights mean we will have a better opportunity to study the Universe we live in, and perhaps spot one of the following monthly highlights:

July 3rd - Earth at Aphelion

Sometimes scientists use strange words to describe astronomical events, and Aphelion is one of them. The term comes from the fact that Earth, like other planets, does not orbit in a perfect circle. Instead it moves in an elliptical path (see image below) around the Sun. This motion means that the distance from the Earth to the Sun changes slightly throughout the year.

Image showing how Earth's orbit is not a perfect circle

Scientists call the closest point perihelion and the farthest point aphelion. In 2006 the actual minimum (perihelion) distance will be 147,103,600 km (91,405,940 mi) on 4th January and the maximum (aphelion) distance will be 152,095,700 km (94,507,890 mi) on 3rd July. That is just a 3% difference in both distance and therefore the size of the Sun, as shown below with the solar images taken at aphelion and perihelion in previous years.

Relative sizes of the Sun when seen at Aphelion and Perihelion

You may think that the Earth being closest to the Sun during the Northern Hemisphere winter would cause it to be warmer, but remember, the seasons and cold weather are caused by the tilt of the Earth toward the Sun, rather than its distance. Note that the distances and dates of Aphelion/Perihelion change from year to year.

The Planets

Click HERE and start the Electric Orrery to see where all the planets are today in relation to each other.

Mercury

Mercury starts the month as an evening object, setting in the west-northwest just 40 minutes after the Sun. Being so close to twilight, the orange afterglow will make it hard to spot. Mercury is currently making its way around between us and the Sun, and will reach inferior conjunction (between us and the Sun) on the 18th July. By the end of the month it will reappear as a morning object. Unfortunately, because Mercury is always low when the Sun is about, it is one of those objects that the Liverpool Telescope cannot safely observe.

Venus

Venus continues to move away from us on the inside track and will remain a morning object and brightest planet for most of 2006, finally passing behind the Sun in late October. Having reached greatest elongation (separation from the Sun) in late March, early birds can spot Venus above the eastern horizon for around two hours before sunrise. Why not use the electric orrery to see how Venus remains a morning object over the next few months.

Mars

Mars remains visible throughout the month but continues to diminish in size and brightness as Earth moves away. It can be found above the western horizon shortly after sunset and sets in the northwest around 11pm. Mars continues to move around behind the Sun and will reach superior conjunction (other side of the Sun to Earth) around mid-October. Now only slightly brighter than stars around it, Mars will be noticeable only by its slightly red hue and the fact that it moves from night to night. This is probably the last month to get any useful images of Mars, and sadly it will be another 12 months before we get any detailed images again.

Jupiter

Jupiter remains the best target to observe after passing opposition (the other side of Earth to the Sun) in early May, although it is decreasing in both size and brightness as we start to move away from it on our inside track. By mid-month, this giant of a gas planet can be found high above the southern horizon just after sunset. Why not use Go Observing to obtain an image of Jupiter and see if you can capture its elusive red spot or other storm cloud features on its surface.

Saturn

Saturn continues to decrease in size and brightness as it starts to position itself to pass behind the Sun on the 7th August. At the start of July, it can be found above the western horizon for 90 minutes after sunset, and sets during twilight. This time moves closer to sunset as the Sun increasingly comes between us. We now have to wait until the end of this year to get a decent image of Saturn again, after it has emerged from behind the Sun as a morning object.

Constellation of the month

Each month, new constellations are highlighted for you to identify in the night sky.

This month we will be looking for the constellation of Bootes, the herdsman, located high towards the zenith or highest point in the sky. Bootes appeared to ancient civilisations as a herder or shepherd, who travelled through the night sky during the late Spring and early Summer. Because of its proximity to another constellation known as the plough, it also became associated with a farmer who ploughs the land during spring.

To modern star-gazers Bootes is more easily recognized as the shape of a kite, with the bright star Arcturus at the point of the kite where the tail is attached.

July Night Sky

Arcturus is a bright red supergiant star in the latter stages of its life, with a diameter nearly 20 times that of the Sun, in addition to being over 100 times brighter. Being only 36 light-years away (fairly close for a star), it appears as the brightest star in Bootes, and, in fact, is the fourth brightest star in the night sky.

Red giants are old stars coming towards the end of their lifetimes. For nearly 10 billion years these stars have generated energy in the centre of their cores through a process known as nuclear fusion, whereby hydrogen is turned into helium and light. To this point, these stars have looked like our own Sun, but when the hydrogen fuel starts to run out they start to swell in size and can expand up to 1000 times their original size (see above). In conjunction with the growth comes a change in colour from yellow to red as their surface cools down. The vast size of these Red Giants means they will appear much brighter than normal stars at the same distance.

After the hydrogen runs out, the star continues to shine through the nuclear fusion of helium. Eventually the helium also runs out and the nuclear processes stop. What follows is a beautiful spectacle known as a planetary nebula, where the star starts to shed the outer layers of its atmosphere towards outer space, as shown in the Hubble Space Telescope image above. What's left is the white-hot core that was at the heart of the original star. This is known as a white dwarf star, which slowly cools down over millions of years and will gradually disappear from our sight.

Mythology: Bootes

There is a lot of mythology surrounding Bootes. One legend sees Bootes as the son of Demeter(Ceres), the goddess of agriculture. Bootes is credited with inventing the plough and was placed in the sky to honour his invention. In another legend, Bootes was the son of Zeus and Callisto, who was transformed into a bear by Hera, Zeus' jealous wife. One day Bootes was hunting and was about to kill his mother, who he saw as a bear, when Zeus took pity and took her to the skies, where she became Ursa Major. In fact, Arctutrus comes from the Greek for 'guardian of the bear' and many times he is seen leading Canes Venatici (hunting dogs) chasing the bears around the pole. In Arab mythology, the circumpolar stars (stars around the pole star) are animals and Bootes is the herdsman. The ancient Egyptians saw the circumpolar stars as evil, and Bootes was seen as a hippopotamus, a creature representing the incarnation of a goddess who keeps the evil stars in check. To the Chinese, Arcturus is one of the horns of the dragon, and when the full moon touches it, Spring and the Chinese New Year begin.