The Night Sky in July 2007

Last quarter Saturday 7th	New Moon Saturday 14th	First quarter Sunday 22nd	Full Moon Monday 30th

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

What's happening in the July night sky?

Strange to think that mid-summer's day is now behind us and the nights are slowly but surely starting to draw in. The weather has been particularly poor last month, so not much chance of observing the night sky. Hopefully the storm clouds will clear for at least some of the late evenings in July, giving us a few glimpses of the odd planet or star here and there, and maybe even one of the monthly highlights mentioned below.

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

July 14th - Venus at its Brightest

The planet Venus reaches its peak brightness for 2007 on the 14th July, as it slowly positions to pass between the Sun and Earth. Try using the Electric Orrery to see how Venus catches the Earth up on its inside (closer to the Sun) orbit. Venus is currently 7 times brighter than the next brightest planet, Jupiter, so should be fairly easy to spot in the western sky shortly after sunset - as shown in the Stellarium image below.

Stellarium map showing position of Venus on 14th July and LT image of Venus on the 1st July

The brightness of Venus is a balance between its apparent size and phase. As it gets closer to us it appears bigger, but because it orbits nearer the Sun, we see less and less of its surface. This is much like the phases of the Moon, and as with the Moon we can get crescent phases for Venus. The phase of Venus can be seen in the Liverpool Telescope image (above) taken on the 1st July. After the 14th, however, the decrease in the amount of sunlight reflected by the phase of Venus overtakes the increase in size due to Venus getting closer. The result being that Venus will start to get dimmer again.

July 20th - Mercury at Greatest (Western) Elongation

At around 4pm on the 20th, Mercury will reach a point known as Greatest Elongation, where it will be at its maximum apparent separation from the Sun. In other words, the angle we measure between Mercury and the Sun will be at its greatest. Greatest separations happen twice during each orbit, once when Mercury is to the East of the Sun and once when it is to the West. Have a look at the diagram below to see what we mean.

Diagram showing the positions of Greatest Elongation

The event on July 20th will see Mercury 20° to the West of the Sun, which means it can be seen in the morning sky for around 30 minutes before morning twilight. Remember that the Sun rises in the East, so anything to the West of it will come over the horizon first. If you cannot picture this, why not try the animations in the Day and Night section of the website.

What are the planets up to?

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

Mercury - emerges from the glare of the Sun as a morning object during the first week of July, after passing inferior conjunction (between Earth and Sun) on the 28th June. It reaches greatest elongation on the 20th (see monthly highlight above) and towards the end of the month, Mercury rises above the east-northeast horizon about 90 minutes before sunrise. Venus - spends its last full month as an evening object as it prepares to pass inferior conjunction (between Earth and Sun) in August. At the start of July it can be found low above the western horizon for just under 2 hours before sunrise, but by the end of the month this reduces to just over 20 minutes, and we will struggle to see the still bright Venus in the glare of twilight. Mars - starts the month rising in the east-northeast around 1.45 am, and by sunrise can be found high above the east-southeast horizon. These times move forward by 75 minutes as we continue to catch it up on our inside track. Mars continues to increase in both size and brightness during the month, making it a good target to observe using Go Observing. Jupiter - can be found low in the southern sky shortly after sunset throughout the month. It sets around 3 am at the outset, but this comes forward by 2 hours by the month's end as we start to move away. Try using the Electric Orrery to see how Jupiter slowly starts to move away from Earth. Saturn - starts the month very close to the planet Venus, and can be found low above the western horizon for around 2 hours after sunset. This reduces to 30 minutes by the end of July as Saturn and the Earth start moving to opposite sides of the Sun ahead of conjunction (where Saturn and the Sun line up) on the 21st August. Sadly, we will not be able to get a decent image of Saturn for another 4 to 5 months.

Constellation of the month

Each month, a new constellation is highlighted for you to identify in the night sky.

This month, we are going to look for the constellation of Hercules, the Greek hero, which is close to the Summer Triangle stars of Vega, Altair and Deneb. We found out about the summer triangle in last month's night sky article. Once you've identified the triangle, high in the southern sky, it's then easy to spot the Hercules constellation because it's just to the right of the star Vega - top right of triangle in the image below.

July Night Sky

Hercules is not the most noticeable of constellations, but is the fifth largest in the sky. Although it has very few bright stars, it contains one of the most spectacular globular clusters that can be seen from the Northern hemisphere, called M13.

M13

M13 Globular Cluster containing over 100,000 stars - © NASA

M13 is home to more than 100,000 stars that have clustered very close together. It is 150 light years across, lies over 20,000 light years away and is over 12 billion years old - that is nearly 3 times the age of the Sun. Back when our galaxy, the Milky Way, first formed it contained thousands of globular clusters, but today less than 200 survive. Most were destroyed by collisions with each other or the centre of the galaxy, where they were pulled apart by gravity and swallowed up.

Mythology: Hercules

Hercules was the Roman name for the greatest hero of Greek mythology -- Heracles. Like most authentic heroes, Heracles was the son of the supreme god Zeus and a mortal woman called Alkeme. Zeus's queen Hera was jealous of Heracles, and when he was still an infant she sent two snakes to kill him in his crib. Heracles was later found alive and happy with a strangled serpent in each hand.

Heracles became arguably the greatest of the mythical Greek heroes, best known for his superhuman strength. In later life he was driven mad by Hera, and in a frenzy, killed his own children. To make up for this crime, he was sentenced to perform a series of 12 famous tasks, or 'Labours', for his cousin Eurystheus, the king of Tiryns and Mycenae. After completing the tasks, all fables in their own right, Zeus thought that Heracles had suffered enough and asked Hera to end her anger toward him, which she did. After death, Heracles was brought to Mount Olympus and was accepted as one of the gods.