The Night Sky in July, 2017

July isn’t a very good month for observing the sky in the Northern hemisphere because of the light nights. But it is often a good time to observe the Milky Way. The term “Milky Way” is used in two different ways by Astronomers. It is the name given to our entire galaxy but it also the name given to the splash of stars which spreads across our sky and looks like spilt milk – hence the name. When I am running observing evenings, people often ask me where the Milky Way is. The answer is that it runs right across our sky from the north through the east to the south. This is shown in the three images below which show the night sky over Oxfordshire at 23:00 on July 1st. The Milky Way is shown in white for clarity. In practice, of course, it is faint and can only be observed on a dark, clearnight. Also it merges with its surroundings and does not have a sharp edge as shown. The first image shows the sky in the north where the Milky Way can be seen running through the constellations of Auriga, Perseus and Cassiopeia. Note that the constellations of Pegasus, Andromeda and the Andromeda galaxy are to the right of it. The second imageshows the sky in the east with the Milky Way running through Cassiopeia, Cepheus, Lacerta, Cygnus, Vulpecula, Sagitta and Aquila. Note that the Milky Way divides into two in the constellation of Cygnus the Swan. The dark region is known as the Great Cygnus Rift and is caused by clouds of molecular dust obscuring our view of the stars. The third image shows the sky in the south and the Milky Way running on through the constellations of Aquila, Scutum, Sagittarius and Scorpius.

When you look at the Milky Way, you are looking directly into the central disc of our galaxy. This explains why it is so rich in stars and other objects. An enjoyable way to observe it on a hot July night is to lie back in a garden chair and scan the Milky Way with binoculars. You will find it a stunning sight! Make sure you have a cool drink or two!

There are planets to be seen in the sky this month. Often there are articles in the national newspapers describing the planets to be seen in the night sky. Many people have told me that they try to read these articles but don’t understand them. This is often because they use terminology which they assume the reader knows. Therefore, before describing the planets in the sky this month, I would like to introduce some of the terminology.

The term “planet” is derived from a Greek word meaning “wandering star”. The ancientGreeks, and other early civilisations, recognised that there was something different about the behaviour of these “stars” even though they couldn’t explain why. We now know that the 8 planets in our solar system are in orbit around the Sun as shown in the image below.

The inner planets are moving at a faster speed around the Sun than the outer planets. Planets such as Mercury and Venus will overtake the Earth on the inside and gradually pull away from it. They then disappear behind the Sun and catch up with the Earth again once they have rounded the Sun. Similarly, Earth will overtake the outer planets on the inside. This is the reason why planets are constantly moving in and out of our night sky.

The planets between the Earth and the Sun (Mercury and Venus) are known as the inferior planets. Planets further away from the Sun than Earth (Mars, Jupiter, Saturn, Uranus and Neptune) are known as the superior planets as shown in the image below.

You will often see in astronomy articles that planets are in inferior or superior conjunction and sometimes at greatest eastern or greatest western elongation. This is illustrated in the image below. When an inferior planet is exactly aligned between the Earth and the Sun it is in inferior conjunction. The planet cannot be seen from Earth at this time since the Sun’s light drowns it out. When an inferior planet is exactly at the opposite side of the Sun to the Earth it is in superior conjunction. The planet cannot be seen from Earth at this time either because it is at the other side of the Sun.

A planet is at its greatest eastern elongation when it is at its furthest point away from the Sun’s eastern side as seen from Earth. Unfortunately, for historical reasons, the Sun’s eastern side is defined as being on the left of the Sun as we look at it. This is shown in the diagram above where the Sun’s eastern side is on the left but the Earth’s eastern side is on the right! Similarly, a planet is at its greatest western elongation when it is at its furthest point away from the Sun’s western side as seen from the Earth. This is on the right! We can see the planet more clearly when it is at its greatest elongation on either side since it is far away from the Sun’s light. The Moon's east and west are on the right and left as we look at it by the way! It is a contentious issue that the Sun's east and west are not the same.

Although this terminology is difficult to understand, the only thing to remember is that when an inferior planet (Mercury and Venus) has an eastern elongation we will see it after sunset in our western sky. When it has a western elongation, we will see it just before sunrise in our eastern sky! As can be seen in the diagram below, this is slightly different from the superior planets. When a planet is to the east of the sun it is said to be in the eastern quadrant. When it is to the west of the Sun it is in the western quadrant. A superior planet can be on the opposite side of the Sun from the Earth when it is in superior conjunction. Or it can be on the opposite side of the Earth from the Sun when it is at opposition. When a superior planet is at opposition we see it at its brightest since it reflects back sunlight from the whole of its disc.

The image below shows how the early astronomers, such as Copernicus for example, would measure the elongation of a star without the benefit of our modern technology. They would align a straight edge along the line between the Sun and the planet and mark their positions. The distance between the marks can be measured and lines can then be drawn from the Earth to the Sun and to the planet resulting in a triangle. The angle between these lines can then be calculated with simple trigonometry. This gives the elongation which is officially defined as the angular separation between the Sun and the planet. It is quoted in degrees. Sorry if you didn't find trigonometry simple!!

I hope this helps a bit with a difficult subject! Please contact me if you have any queries.

This month there will be a Full Moon on July 9th and a New Moon on July 23rd. The end ofthe month will therefore be better for stargazing than the beginning but of course the sky never goes completely dark in July. The superior planets Saturn and Jupiter will still be putting on a good display in the southern sky but are low on the horizon. This can be seen in the star chart below which shows the sky in the south and southwest over Oxfordshire at23:00 on July 1st. Saturn is in the constellation of Ophiuchus. To the right of it is the top of the constellation of Scorpius with its bright red supergiant star Antares. To the right of that is the constellation of Libra with its bright star “Zubeneschamali”. It takes practice to pronounce this name (it is almost as difficult as "Supercalifragilisticexpialidocious")! Zubeneschamali is a very hot, young star with a temperature of 10,143 degrees compared with the Sun’s temperature of about 6,000 degrees. It is 5 times the size of the Sun and is 160 light years away. To the right of this you can see the constellation of Virgo with its bright star Spica and the Moon and Jupiter nearby.

Venus is back in our skies but now it is in the eastern sky before dawn. The star chart below shows it at 4am on July 27th in the constellation of Taurus the Bull shown highlighted.

You may remember that last winter Venus was brilliant in our western sky after sunset, Now we know from the discussion above that it was in the eastern quadrant of the Sun. Then it disappeared since it passed between the Earth and the Sun and was drowned out bysunlight. Since it moves faster than we do, it gradually pulled ahead of the Earth. Now it has become visible again in the western quadrant of the Sun in our morning sky.

Mercury reaches its greatest eastern elongation from the Sun on July 30th and will be visible in our western sky. The star chart below shows the sky in the west at 19:00 on July 30th. You can see that Mercury is very low on the horizon but you may be able to spot it with binoculars. Please don’t attempt this until the Sun has set.

Now to the Southern Hemisphere!

What's Up in the Southern Hemisphere?

It is mid winter for you in the Southern Hemisphere and a great time for observing. All of the preceding discussion on the terminology used to describe the positions of the planets applies to you as well. But you also have the great advantage to observe the planets and the richness of the Milky Way more favourably than we can this month. The star chart below shows the sky in the west over Sydney at 23:00 on July 1st. Here you have a wonderful view of Jupiter close to the Moon in the constellation of Virgo. To the left of Jupiter you can see the Sombrero galaxy M104 (Messier object 104). Below Virgo and tothe right you can see the constellation of Coma Berenices. This region is known as the Realm of Galaxies. It is rich in Messier objects such as the Blackeye galaxy, M64.

If you look in the east at the same time you get a wonderful view of Saturn in the constellation of Ophiuchus. Above it is the constellation of Scorpius and below it to the right is the constellation of Sagittarius. This region is very rich in Messier object. You arepositively spoiled for observing and imaging possibilities this month! I need say no more!

Until next month, happy stargazing and dark skies!

Valerie Calderbank FRAS