Mercury will be  the most difficult to see, being only about 20 degrees away from the Sun at the beginning of the week, and 23 degrees away from the Sun by end of week. However, it is a bright magnitude 0 object.
Venus will be the brightest at magnitude -4, and will be about 20 degrees away from Mercury throughout the week. Note that Mercury will be about halfway between Venus and the setting Sun.
Mars is about 12 degrees away from Venus, and easily found by noting its reddish color.
Saturn is close to Mars at 5 degrees away, with Mars closing in on Saturn as the days progress. By month’s end, they will be less than 2 degrees apart.

July 23rd, 2010, 9:00 PM MDT

By finding these four planets, one can easily see the ecliptic plane, especially when extended to the Moon over in the southeast this week. The ecliptic is formally defined as the plane generated by Earth’s orbit, or as the path of the Sun across the sky. It is also known as the plane of the solar system, and all major planets and Earth’s Moon are found near the ecliptic.

The ecliptic is defined as the path of the Sun against the background stars, or the plane generated by Earth’s orbit. However, both of these statements are not very visually helpful. It’s difficult to see any background stars when the Sun is up, and it’s tough to find Earth’s orbit when we’re on it. However, the ecliptic is also described as the plane, or ‘flatness,’ of the solar system. All the major planets orbit the Sun relatively close to the ecliptic plane. There are also several bright stars that lie very close to the ecliptic. This week, we have three planets, the Moon and three bright stars that can help us see the ecliptic plane across the night sky.

Friday the 14th, 10 PM, MDT

When viewed at 10 pm, MDT on Friday, May 14, 2010:
Venus is low on the west-northwest horizon at a bright magnitude -4. Venus sets just after 11 pm.
Mars is high in the southwest at 45 degrees above the horizon. It has a reddish appearance and is a descent magnitude +1.
Regulus, the bright blue star in Leo, is just over 10 degrees east of Mars. Regulus is often referred to as an ecliptic star, because it is so close to the ecliptic.
Saturn is due south at 10 pm, 50 degrees above the horizon, at magnitude +1.
Spica, the bright blue star of Virgo, is also close to the ecliptic and is 34 degrees above the south-southeast horizon at the date/time specificed.

Sunday the 16th, 10:30 PM, MDT

If that’s not enough, as Venus sets Antares, the red heart of Scorpius, rises in the southeast. Antares is a +1 magnitude star 5 degrees south of the ecliptic.
But wait, that’s not all. A young crescent Moon appears near Venus on the evenings of the 15th and 16th, also on the ecliptic plane.

I suggest that you do this more than one evening, just to make sure you know what you’re looking at. Once you recognize the ecliptic, watch the waxing Moon for several days, preferably at the same time each evening, and you’ll see it march eastward along the ecliptic.

(images generated with Starry Night Pro, v6.3.9)

The other morning I noticed that, as I’m facing south, Scorpius is nice and bright and easy to spot. Now, according to tradition, I’m ‘a Scorpio,’ but that discussion will have to wait [tease, tease].

Looking south at 6:30 AM, MST, from the Salt Lake valley area

Scorpius is usually thought of as a summer or autumn constellation, when it is visible in the warm, evening skies after sundown. But, the sky we see at night in August is the sky we see pre-dawn in February. And so it is with Scorpius. The familiar fish hook, or ‘J-shape’ of Scorpius is dominated by the bright red star Antares.

Zoomed in view showing Antares and M4

Antares is a respectably bright magnitude +1 star about 600 light years away. Antares is a huge red giant star, so big, in fact, that if it were placed at the center of our solar system, its outer edge would lie about halfway between Mars and Jupiter (in the asteroid belt). Its size is 750 or so times the diameter of our sun, but only 15 times as massive, making Antares a very low-density star. Indications are that this is a star near the end of its life and would probably go supernova in the astronomically-speaker near future. Antares is also very close to the ecliptic.  With it’s brightness, location and brilliant red color, Antares can easily be mistaken for the planet Mars. In fact, the name Antares means ‘like Mars.’ Mars will be approaching Antares later this year (October), but the two will quickly be lost in the sunlight as the Sun passes by Antares in late-November to early-December.

About 1.5 degrees to the right of Antares is a star cluster, M4, known as the ‘Cat’s Eye’ cluster. This cluster is easily visible with binoculars or a small telescope at magnitude +7.5. M4 is only 7200 light years away from us, making it one of the closest clusters to the solar system.

Antares and M4 are due south around 6:30 a.m. right now, and well worth a look.

However, there are objects outside the solar system that can be seen pretty close to the ecliptic plane. Saturn is currently in Leo, the Lion. The brightest star in Leo, Regulus, is an ‘ecliptic star,’ being less than 1° off the ecliptic. Regulus is easy to see as a 1st magnitude star about halfway up the sky from the southeastern horizon, rising about 1 1/2 hours before, and 17° away from, the planet Saturn.

About 20° away from Regulus, and only about 1° off the ecliptic is M44, the Beehive Cluster in Cancer. M44 is a bright, 3rd magnitude, open cluster easily visible on a clear, dark night with the unaided eye, looking like a large cotton ball (better viewed with binoculars). M44 rises about 2 hours before Regulus, and is 2/3 the way up the south-southeast horizon at the designated time.

And if that’s not enough to find the ecliptic, over in the west is M45, the Pleiades, only 3° off the ecliptic. The Pleiades is in Taurus and is an easily visible open cluster often confused with the Little Dipper, because of the distinctive shape of the 6 brightest stars. At the specified time, M45 is about 1/3 the way up from the western horizon.