July 2010 Events – a really cool gathering of planets – where, when, and why

This is how the western sky should look on July 27, 2010, about forty minutes after sunset, from mid-northern latitudes. Venus should be easy. Use binoculars to help spot Mars and Mercury. On this date Mercury will make a neat double "star" with Regulus - the planet being brighter by about a full magnitude. But don't wait until July 27th - these planets put on a good show all month long! (Click image for larger view. Prepared from Starry Nights screen shot.)

• How to find the planets
• Planets in early July
• Planets in the middle of July
• Planets at the end of the month
• July calendar

This bright grouping of planets will make a great display in our western sky near the end of the month – but don’t wait until then to look for them. The fun is seeing them majestically draw together and that’s an all-month event! What’s more, it should give you an excellent sense of the plane of our solar system! I love events like this because they’re accessible to everyone, and if you approach them correctly you can almost hear the proverbial music of the spheres as gravity does its magic and the sky presents us with a changing tableau in four-part harmony. In July that tableau will include close encounters between planets and a bright star, as well as other planets and the Moon. Essentially, this is a show to enjoy any evening this month by stepping out about 45 minutes after sunset and looking west. By the end of the month the planets will be so low, however, you’ll need an unobstructed western horizon and clear skies to pick them out, even though they are all reasonably bright. Binoculars will help!

Unless the Moon happens to be in that section of sky – as it will in mid-month, the first thing that should catch your eye is brilliant Venus. Nothing outshines it except the Moon and Sun, and it should come into view half an hour or so after sunset. At this point sit back in your lawn chair and enjoy. As the sky continues to darken, Saturn will probably be the next to pop into view, followed by Mars. (Scanning for it between Venus and Saturn with binoculars will help.) At the beginning of the month all three are in a line that stretches over almost 40 degrees (four fists) of sky. All month these three will be drawing closer to one another until near the end of the month they are less than 10 degrees apart – a gathering so tight you could hold it in the bowl of the Big Dipper.

Enter Mercury

But wait – there’s more. In the last half of the month you should also scan near the horizon for Mercury. It will be quite bright – magnitude 0 – but despite its brightness may be difficult to see because it is so low and getting lost in the strong twilight. As twilight deepens it will be easier to see, but it will also be getting lower – so consider finding it a challenge. (This time around southern hemisphere observers have a better angle on Mercury and should find the fleet-footed planet easier to find. Just identifying these “wandering stars” is an accomplishment, but what makes the experience richer is to be able to envision why we are seeing them this way. It helps to picture the solar system as a disc, with all the planets on roughly the same plane. The line you see them in slanting towards the horizon is that plane. But when you take a look at the solar system from above it all becomes clearer. So in the charts below I’ve combined the sky view with an inset that shows you a view of the bright planets as seen from an imaginary vantage point above our solar system. (These inset charts are derived from online Orrery at Solar System Live, a web site I urge you to visit.) If you can see the relationship between the two charts – the one showing what we see and the other why we see it this way – try to carry that mental image out with you as you look at the real thing. Imagine where the Sun is and each planet – including Earth – are. In this way you can take your observing to a new level. OK – one essential – after reading the caption for each chart, click on the image and get a much larger view. It’s next to impossible to tell much from these small images.

Early July - Start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4 and about a fist and a half above the western horizon. Saturn should be the next easiest at magnitude 1.1 and about twice as high as Venus. Mars is the dimmest of the three, but you should be able to see it. Binoculars will help. Once you locate Venus just sweep up and to the south. Halfway between Venus and Mars you should see the guidepost star, Regulus, almost exactly as bright as Mars, but appearing a tad dimmer because the twilight there will be stronger. Be sure to click on the above image to see a much larger chart. Note in the inset how Venus, Mars, and Saturn are in a straight line, but Mercury is still quite far away. Jupiter is a late night/early morning event this month, rising in the east after midnight at the start of the month, but about two and a half hours after sunset by the end of the month. It shows in the Orrery view but is not part of the western sky show. (Chart prepared from Starry Nights Pro Screen shot - inset from the Solar System Live.)

Mid July - Again, start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4. Saturn should be the next easiest and with the crescent Moon and Venus should form a shallow triangle. Mars should be nearest the Moon. Binoculars will help. Be sure to click on the above image to see a much larger chart. Note in the inset how Mercury is getting closer to Venus, but it is still very difficult to see in the evening sky. However, it will get easier day by day as we move towards the end of the month. (Chart prepared from Starry Nights Pro screen shot - inset from the Solar System Live.)

Late July - Again, start your search about 45 minutes after sunset. The first planet you'll pick up is Venus, at magnitude -4. Look down and to the right for Mercury. At this stage binoculars will be a big help. Saturn and Mars are now close together and both have drawn closer to Venus. The inset shows the relationship from a perspective above the Solar System. Notice that speedy Mercury - with its smaller orbit - is now very close to Venus. Be sure to click on the above image to see a much larger chart. (Chart prepared from Starry Nights Pro screen shot - inset from the Solar System Live.)

I think that checking these planets any evening this month will be fun, but there are some special dates when they have close encounters with bright stars that are particularly interesting. You’ll find these in the chronological event summary that follows.

July 4 – Last quarter Moon

July 6 – Earth at greatest distance from Sun – 94,508,000 miles – feel any cooler? (No, distance has nothing to do with seasons 😉

July 8 – Do the Pleiades look small to you? Go out about 3:30 in the morning and look at the crescent Moon low in the east. That’s the Pleiades just above it and the two should show really nicely in the same binocular field. Which is larger? The little star cluster, or the Moon?

July 9, 10 – Ever wonder what a difference of about five magnitudes amounts to? Take a look at Venus about an hour or so after sunset tonight. It has a close encounter with Regulus. Venus is -4, Regulus 1.3 – they’ll both fit easily in the same binocular field.

July 11 – New Moon

July 15 – Crescent Moon forms a nice triangle with Saturn and Venus, while being nearest to Mars.

July 18 – First quarter Moon

July 26 – Full Moon

July 27 – Mercury squeaks by Regulus – less than half a degree separates them – making a nice binocular “double star.”

July 29, 30 – Mars and Saturn should make a real nice site in binoculars. Being so close to the horizon both may show a lot of false color, but do you notice the color difference of the two planets?