July 2018

Planets to look for

Attention all Mercury fans! July is the best time to see the innermost planet in the evening sky during 2018. For most of the month it will be setting after twilight ends. If you need a bit of help spotting it, the Moon will be above and to the right of it on the evening of the 15^th. Venus is the next planet, shining very brightly in the northwest, still. Not much has changed here, but look out for the Moon next to it on the evening of the 16^th.

Jupiter won’t appear to move very far, either, during July, as it finishes its annual loop of retrograde motion near Zubenelgenubi (alpha Librae) on July 11th. It is now high overhead at the end of twilight. Saturn is also easily visible in the east near the lid of the Teapot asterism of Sagittarius. Look for it above the Moon on the 25^th of July.

The “star” of the show this month is Mars, as it reaches it opposition at the end of the month. This is the most favourable opposition since 2003, so it is looking particularly bright in the sky – even outshining Jupiter! It is moving through its own retrograde loop above Capricornus this year, and the Full Moon joins it on the night of opposition (See the feature article for this month for more information).

If you have been paying attention, you may have realised that all five planets that you can see with the unaided eye will be visible in the evening sky in the second half of July (Sorry morning people! You’ll have to wait until August for Mercury to return.) Starting in the west-northwest there is Mercury, then Venus, Jupiter overhead, Saturn high in the east then Mars rising low in the east. It’s not often the planets are bunched together on one half of the sky. Part of the reason is that Jupiter and Saturn are creeping towards their once-in-20-years conjunction in 2020, and Mars just happens to be hanging around nearby!

All five of the bright planets will be visible along the ecliptic in the evening during July. The diagram above shows the position of the Moon near Jupiter on the evening of July 21^st.

Constellation of the month

Hercules

Hercules is the fifth largest constellation in the sky. Named after the mythological Greek hero famous for his great strength, he may also represent the ancient Sumerian hero Gilgamesh. The constellation depicts a strong man kneeling on the head of Draco the Dragon. Although the constellation is in the northern sky as seen from Australia, it still rises far enough above the northern horizon to be easy to see.

A prominent feature of Hercules is the asterism known as the ‘Keystone’. An asterism is a pattern of stars that is not a constellation and is often a smaller part of a recognised constellation. The four stars that mark the lower body of the man form the same shape as a keystone in the top of a stone archway. The keystone holds the whole arch together and stops it collapsing.

The constellation Hercules can be seen towards the north during winter in the southern hemisphere

When it comes to globular clusters we are pretty spoilt for choice in the southern hemisphere: we have omega Centauri, 47 Tucanae, M22 and the Starfish cluster (NGC 6752 in Pavo). North of the equator it’s M13 at the top of the list.

M13 is a globular cluster that is large and bright enough to be seen with the unaided eye from a dark location.  Estimated to be 25,200 light years away, it’s still relatively close for a globular cluster.  It is found along one side of the ‘Keystone’ asterism in Hercules.  In binoculars it looks like a round fuzzy patch, so don’t confuse it with a comet!

Globular clusters are large clusters of hundreds of thousands of stars that are probably left over bits from when the galaxies formed.  Most of the stars in the cluster are very old. Because the clusters typically orbit around the outside of the galaxy they retain a round, globular shape, which is how they got their name. We can see around 120 globular clusters that belong to our galaxy from Earth, and we can see thousands around other large galaxies beyond.

M13 is a large, bright globular cluster in Hercules.

Credit: N.A.Sharp, REU program/NOAO/AURA/NSF

A Moon and Mars Double Act

On the night of 27^th of July Mars will reach opposition – it will be opposite the Sun as seen from the Earth. This is also the best time to be viewing Mars, as the distance between Earth and Mars will be minimal, around 58 million km or so. This is the most favourable opposition since the really close one of 2003, 17 years ago, so it’s an event to get excited about, even though we’ve got active robotic rovers on the surface of Mars now.

Mars only comes into opposition once every two years and 50 days, and even then it may not be particularly close. Since Mars has the most elliptical of all the planets in the solar system, it can be as far away as 101 million km at a bad opposition, or as close as 55 million km at a good one. That 56 million km difference really matters when you are trying to make out some detail in the surface features in Mars in a telescope!

Sadly, none of us may get to see anything this time – a dust storm started on the 30^th of May that kept growing and by the 12^th of June was observed to be encircling the entire planet. These kind of planet-wide dust storms are not unknown on Mars, but there hasn’t been one this big since 2001. The Curiosity rover is fine, since its nuclear powered, but there are concerns for the solar-powered rover Opportunity, which has gone into hibernation mode and stopped communicating with Earth. All we do now is wait, listen, and watch…

A self-portrait by NASA’s Curiosity rover taken on Sol 2082 (June 15, 2018). A Martian dust storm has reduced sunlight and visibility at the rover’s location in Gale Crater. A drill hole can be seen in the rock to the left of the rover at a target site called “Duluth.”

Credits: NASA/JPL-Caltech/MSSS

The 27^th of July also happens to be the night of a total lunar eclipse! This is a particularly “deep” eclipse, in that the centre of the Moon passes close to the centre of Earth’s shadow. The only downside is that it happens in the early hours of the morning! It is on a Saturday morning, however, which could help some people.

If you have watched a few lunar eclipses you’ll know there is a particular sequence of events that occur: The Moon first enters the penumbra, the partial shadow of the Earth, before reaching the umbra, the full shadow of our planet. The penumbral region gives a gradual shading of the Moon from nothing down to dark grey, but once the Moon slides into the umbra, the real action starts. The Moon will look very dark at first, but as more of it goes into the umbra, the coppery-red colour of the lunar eclipse will start to appear. This is caused by the Sun’s light bending around the Earth in the atmosphere. There haven’t been any major volcanic eruptions recently, so the red colour should be quite apparent.

So what time should you set your alarm for? If you are in WA, the Moon enters the umbra at 3.30am, mid-eclipse is at 4.22am, and it will have left the umbra by 5.14. In the eastern states, mid-eclipse is 6.22am. Full details can be found here: NASA Eclipse Site, EclipseWise.com, TimeandDate.com  This is the last total lunar eclipse we will be able to see from Australia until May 2021.

The position of the Moon near Mars on the evening of 27^th July, 2018