Jane Houston Jones is an amateur astronomer and the senior outreach specialist for the Cassini mission to Saturn.
Are you eager to see the annual Perseids meteor shower tonight? You'll have to wait until near midnight to see it, so why not pass the time by viewing Venus, Saturn and Mars right from your doorstep? Step outside for the planetary warm-up act just as soon as the sun sets. (Viewing times will be best over the next week. By August 20, the planets set lower on the horizon and are harder to see.)
All you have to do is look towards the west for bright Venus to appear. Now hold your clenched fist up to the sky, covering Venus. To the right of Venus, about half of a clenched fist away, is a second planet: That's Saturn! And to the upper left of Venus is another planet: Mars!
That's not all you'll be able to see. Look below Venus for the slender crescent moon. If you don't see the moon, look again on the night of Friday, August 13 -- it will be a larger crescent to the left of Venus.
Though the three planets appear together in our line of sight, they are really far apart from each other. Mars is about 300 million kilometers (about 185 million miles) from Earth, while Venus is 112 million kilometers (about 70 million miles) away. Saturn? It's 1,535 million kilometers (about 954 million miles) from Earth. And finally, the moon is only 363 thousand kilometers (about 225 thousand miles) away. It's fun to compare the size of the moon and Mars, especially if you received that annual email incorrectly stating that Mars will be as big as the moon this month.
The end of 1609 and the first months of 1610 mark the beginning of modern astronomy. 400 years ago today, January 7th, Galileo Galilei looked up towards the constellation Orion. He aimed his telescope at an object brighter than any of the surrounding stars - the planet Jupiter.
The view through his telescope startled him. He did not see only one object, but rather, one large world, with four smaller objects nearby.
These four objects are the moons we now call Io, Europa, Ganymede and Callisto.
Galileo wrote in his book Sidereus Nuncius, which was published in 1610 the following words:
"I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day . . . On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavons through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . .When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night."
"I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter."
On the 400th anniversary of their first sighting, use a pair of binoculars to spot Galileo's four tiny moons directly next to the planet. On the evening of January 7 look to the southwest after sunset. Europa and Ganymede will appear to the upper left and Io and Callisto on the lower right of Jupiter.
Letters with translations http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html
Updated Aug. 26, 2010
If you're like me, you may have received an e-mail this summer telling you to go outside on August 27 and look up in the sky. The e-mail, most likely forwarded to you by a friend or relative, promises that Mars will look as big as the moon on that date and that no one will ever see this view again. Hmmm, it looks like the same e-mail I received last summer and the summer before that, too. In fact this same e-mail has been circulating since 2003, but with a few important omissions from the original announcement.
I'm Jane Jones, an amateur astronomer and outreach specialist for the Cassini mission at Saturn, and I'm here to set the record straight on when and how you can actually see Mars this month.
1. How did the "Mars in August" e-mail get started in the first place?
In 2003, when Mars neared opposition -- its closest approach to Earth in its 22-month orbit around the sun -- it was less than 56 million kilometers (less than 35 million miles) away. This was the closest it had been in over 50,000 years. The e-mail that circulated back then said that Mars, when viewed through a telescope magnified 75 times, would look as large as the moon does with the unaided eye. Even back in 2003, to the unaided eye, Mars looked like a reddish star in the sky to our eyes, and through a backyard telescope it looked like a small disc with some dark markings and maybe a hint of its polar ice cap. Without magnification, it never looked as large as the moon, even back in 2003!
2. Can the moon and Mars ever look the same size?
No. The moon is one-quarter the size of Earth and is relatively close -- only about 384,000 kilometers (about 239, 000 miles) away. On the other hand, Mars is one-half the size of Earth and it orbits the sun 1-1/2 times farther out than Earth's orbit. The closest it ever gets to Earth is at opposition every 26 months. The last Mars opposition was in January and the next one is in March 2011.
At opposition, Mars will be 101 million kilometers (63 million miles) from Earth, almost twice as far as in 2003. So from that distance, Mars could never look the same as our moon.
3. Is Mars visible in August 2010?
Mars and Saturn made a dramatic trio with brighter Venus this month. Skywatchers enjoyed seeing the three planets closely gathered on the 12th and 13th with the slender crescent moon nearby. On the 27th, you'll see Venus shining brightly in the west. If you look above Venus, you may find faint Mars. Saturn is barely visible above the horizon, getting ready for its solar conjunction next month.
4. Can I see Mars and the moon at the same time this month?
Both the moon and Mars were next to one another on the 12th and 13th, but now you can see both planets a few hours apart. Look for Mars in the west at sunset, and watch the moon rise in the east a few hours later. On August 26th and 27th you can see the nearly full moon rising in the east at about 10 p.m. The bright planet below the moon on the 26th is Jupiter! On the 27th, the moon is to the left of the planet.