What a Sight! The Moon and Three Planets

by Joe Rao, SPACE.com Skywatching Columnist

Date: 18 August 2006 Time: 06:14 AM ET

What a Sight! The Moon and Three Planets

SKY MAP: The scene at 6 a.m. local time on Aug. 22 from mid-northern latitudes.
CREDIT:

Three planets and a crescent Moon will be putting on an ever-changing display in our morning twilight sky this week.

Anybody who looks low toward the east-northeast about an hour before sunrise will immediately see Venus, which has been a fixture in the morning sky since the end of January.

As it usually does, Venus is shining brilliantly (this week it's at magnitude -3.8). Since the beginning of July it has been rising out of the east-northeast sky just as dawn breaks, about 1-½ to 2 hours before the Sun. But through the remainder of August it loses altitude noticeably as it begins its plunge back toward the Sun.

Joining Venus this week is the planet Mercury. This speedy planet is having a fine morning apparition in August, having already reached greatest western elongation-its greatest angular distance-19 degrees from the Sun back on Aug. 7. At that time it was rising nearly 90 minutes before sunup and shining at magnitude +0.1, similar in brightness to the bright star Capella. Since then, Mercury has been slowly dropping back toward the Sun but as if to compensate, it has been growing progressively brighter.

The third planet of the group is Saturn, which passed through solar conjunction on Aug. 7. During the coming week, you should use Mercury and the brilliant Venus to guide you to Saturn as it begins to emerge into the morning sky.

On the 20th, Mercury will be positioned just over 1-degree directly above Saturn, both planets positioned below and to Venus' left. A narrowing crescent Moon will slide past Venus on the mornings of the 21st and 22nd. That same sliver of Moon-less than 36 hours from new phase-will be passing well above and to the left of Saturn on the morning of the 22nd. By then, Mercury will have brightened to magnitude -1.4, matching Sirius (the brightest star in the sky). [Sky Map]

The next morning, Mercury, Saturn and Venus will appear almost equally spaced apart from lower left to upper right along a diagonal line running about 8-degrees in length. For comparison, your clinched fist held at arm's length measures roughly ten degrees in width. Mercury will then depart the scene, leaving Venus and Saturn behind and disappears into the dawn, passing superior conjunction on Sept. 1.

On the 26th, Saturn will sit half a degree below and slightly to the left of Venus. That's approximately the apparent width of the Moon. By the following morning it will be half of a degree above and slightly to the right of Venus. But compared to the dazzling morning star, Saturn will appear only 1/48 as bright.

By the end of August, Venus will be rising just over an hour before the Sun. But it's also rapidly sliding toward the Sun and by the end of September it will be rising only about a half-hour before it and despite its great brilliance it will finally become lost in the glare of the bright morning sky.

As for Saturn, it will continue to climb progressively higher in the east-northeast sky as we transition from August into September. Shining at magnitude +0.5, it rises around 4:45 a.m. local daylight time at the start of September; 3 a.m. by the 30th. Any telescope magnifying more than 30-power will reveal the famous ring system which currently is tipped nearly 17-degrees toward Earth.

Lastly, keep in mind that when you are looking at this gathering of the Moon and three planets in this week's morning sky that in actuality, they are all at varying distances from our Earthly vantagepoint here on the Earth. The Moon is, of course, the closest at roughly 238,000 miles. Mercury comes next at about 114 million miles, followed by Venus at around 150 million miles. Finally, most distant of all, comes the ringed wonder, Saturn, at 943 million miles.

Basic Sky Guides

Full Moon Fever
Astrophotography 101
Sky Calendar & Moon Phases
10 Steps to Rewarding Stargazing
Understanding the Ecliptic and the Zodiac
False Dawn: All about the Zodiacal Light
Reading Weather in the Sun, Moon and Stars
How and Why the Night Sky Changes with the Seasons
Night Sky Main Page: More Skywatching News & Features

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

DEFINITIONS

1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).