Sky Roundup: June, 2007

So, Where’s My Planet?

Ask any amateur or professional astronomer:  What’s your favorite planet?  No doubt you’re likely to get a variety of responses.  Just about every planet might be named a favorite, including poor ol’ Pluto, which was recently reclassified as a so-called dwarf planet. 

In return, that astronomer ought to ask you for your own favorite.  And once you let your own preference be known, you then might ask:  So, where’s my planet?  Finding planets in the sky may take some practice, but keep in mind a few traits about the worlds orbiting our Sun and you’ll know their likely location with less trouble.  

Planets may appear like stars:  they shine brightly in the sky, especially Venus and Jupiter.  But planets shine by light reflected from the Sun, not by their own light.  And since they are closer to us than the stars, they usually shine brighter than the stars.  (Here the word “usually” means that distant planets, such as Neptune and Uranus, would indeed appear less bright than most stars.)  And, the nearer, brighter planets will not twinkle as much as the distant stars.  

That twinkling is due to heat rising through the atmosphere, which causes air to ripple as heat rises.  It’s that rippling of air, not the heat itself, which one sees as twinkling (also known as scintillation).  In addition, it’s the point of light that a star’s distinct appearance which makes it appear to twinkle.  That is, a point source to our eyes is easily made to scintillate by rising heat; but, an extended object, such as a planet, has an area, albeit very small, over which the scintillation is spread.  So, a planet appears to twinkle little (or not at all, depending on your vision) when compared to stars. 

By the way, planets still can appear to twinkle, but only if they’re seen in the sky close to the horizon.  So, when one sees a planet level with the treetops, one really looks at the planet at a low angle and hence through both more air, which enhances the twinkling. 

Venus at greatest western elongation	Mars at opposition
Conjunctions with inferior and superior planets These diagrams show examples of two of the more widely seen planetary conjunctions. The diagram on the left shows Venus, an “inferior” planet, at the position in its orbit to make its greatest (or most elongated) angle in the sky from the Sun as seen from Earth. When seen in this configuration, Venus rises before the Sun, and so is seen in the morning sky west of the Sun, hence the name “greatest western elongation”. When Venus sets after sunset and is seen in the evening sky east of the Sun at its same greatest angle, then it’s at its “greatest eastern elongation”. Venus also shines brightest around this time as well. The diagram on the right shows Mars, a “superior” planet, at its position in its orbit to make the configuration known as opposition. When seen in this configuration, Mars appears opposite the Sun in our sky, hence the name “opposition”. Other superior planets, such as Jupiter and Saturn, are also best seen in this configuration and are brightest at this time since they are closer to Earth at opposition than at other times.

Now, finding the right planet in the sky can be more involved, but not troublingly so.  And, knowing its place in the sky can tell you where it is in its orbit around the Sun, and vice versa. 

For example, distant planets like Neptune and Uranus revolve slowly around the Sun, and hence will occupy the same part of the sky for years.  So, if you consult a sky chart or almanac for their location, you can usually return to that part of the sky for a glimpse (with a telescope) of these distant, slow-moving worlds.  Planets closer in, such as Mars, Jupiter, and Saturn, will remain in the same part of the sky for months at a time, but, too, will gradually move against the background of stars.  Planets closer to the Sun itself, such as Mercury and Venus, swing between morning and evening skies within a matter of months, either appearing before the Sun rises or just after it has set, since these two planets don’t stray very far from the Sun.

To decipher all of these planetary motions, and to determine whether or not you’ll find your favorite, keep in mind that the Earth moves as well; so, we must consider a moving platform in space against which we see all else.  To make things easier, though, there is a set of conventions, used for centuries, that helps to identify planetary positions.  One basic assumption to this set of conventions is the observation that when planets line up in the sky, that alignment is called a conjunction.  In fact, when a planet lines up with another planet (or with the Sun), the planets themselves in outer space are indeed very far apart yet may appear close together in the sky. 

Conjunctions of planets occur fairly frequently.  On some occasions one may need a binocular or telescope; more often, many are visible with the naked eye.  For example, of the former, a close conjunction of Mars and Neptune (separated by 1°) occurred last March 25 in the wee hours and would have required a telescope to view it; of the latter, a very close conjunction of Venus and Saturn (separated by less than 1°) will occur on July 1 in early evening (for us in the Cleveland area) and will be plainly visible with eyes alone.  (But you still may want to aim a telescope at this particular conjunction.  You’ll see the crescent phase of brilliant white Venus in the same field of view with the fine rings and pale yellow globe of dimmer, more distant Saturn!)

Pairings of planets, and even alignment of several planets, are indeed pretty to observe, but they have no other significance on our lives here on Earth.  That is, conjunctions of planets do not exert any supernatural influence on earthly events.  Simply put, conjunctions can be useful to explain where to find planets in the sky.

To understand the types of conjunctions, we must learn a little vocabulary.  For example, it’s customary to refer to the inner planets, Mercury and Venus, as inferior planets.  By “inferior” this doesn’t mean that those planets are substandard; it simply means those planets orbit closer to the Sun than does the Earth.  By contrast, outer planets, such as Mars, Jupiter, and Saturn (which were all known since antiquity) and both Uranus and Neptune (which were not), are called superior planets, because they occupy orbits beyond Earth’s distance from the Sun.

When an inferior planet lines up between the Sun and us on Earth, then that’s known as an inferior conjunction, since the conjunction involves an inferior planet.  However, we don’t generally see that particular conjunction due to the bright glare of the Sun.  Only when the inferior planet lines up between us and the Sun might we see this conjunction as the inferior planet passes in front of the disk of the Sun.  This is then called a transit.  A transit for the inferior planet Mercury is fairly rare and happens only thirteen times (on average) per century.  More exactly, it can presently occur only in early May or early November when Mercury crosses Earth’s orbital plane and when that crossing is visible.  The last one occurred in November 2006 but wasn’t seen in Cleveland; the next will happen in May 2016.

For the planet Venus, transits are even rarer.  A transit of Venus occurs (on average) less than thirteen times per millennium!  And when transits of Venus do occur, they generally happen in pairs, with individual transits in the pair separated by eight years, and presently can only occur in early December or early June.  (To date, only six transits of Venus have been observed since the invention of the telescope.)  The last transit happened on June 8, 2004 and was visible at sunrise across most of the United States; fortunately, the next one will occur on June 6, 2012 and will be visible, already in progress, at sunset from Cleveland.  (Make plans to view it since the next transits of Venus won’t occur until December of 2117 and 2125!)

By contrast, if that same inferior planet is found on the far side of the Sun, then that’s known as a superior conjunction.  Again, we cannot see the planet since it is on the other side of the Sun, and therefore hidden, from our vantage.

Inferior planets are best seen when they make their largest angle in the sky from the Sun, depending on where the planets and Earth are in their respective orbits.  That angular distance for Mercury is between 18° and 28°; that angular distance for Venus is between 45° and 47°.  It also happens that Mercury and Venus are brightest when they appear near their maximum angle from the Sun as seen in our sky, a position known as an inferior planet’s greatest elongation.  Through a telescope, an inferior planet at greatest elongation appears as a crescent shape, much like a waxing or waning crescent phase of the Moon.

Mercury or Venus in the morning sky (that is, as seen in the east) before sunrise at its maximum angle is called greatest western elongation, because either planet is actually west of the Sun as we view it.  Conversely, if Mercury or Venus are seen in the evening sky (that is, visible in the west) after sunset, then its maximum angle is called greatest eastern elongation, because either planet is actually east of the Sun as we view it.  Around these times of greatest elongation, inferior planets shine with greatest brilliancy.  On June 8, Venus appears as a bright beacon in the west after the Sun sets when it’s seen at its greatest eastern elongation. 

When a superior planet lines up with the Sun, it does so either as a superior conjunction, that is, on the other side of the Sun from our vantage, or, in opposition, that is, with us on Earth between the planet and the Sun.  In this latter configuration, the planet is seen in our sky opposite the Sun.  So, a superior planet at opposition rises as the Sun sets and remains in the night sky all night long till dawn.  This is the best time to view superior planets as they are at their brightest.  Through a telescope, superior planets at opposition appear fully lit and round.

One final configuration of note is a superior planet seen at a right angle to the Sun; this is known as quadrature.  That is, the superior planet is seen due south (for us in the Northern Hemisphere) at either sunrise or sunset.  If the superior planet is seen due south in the morning sky just before sunrise, then the planet configuration is called western quadrature, since the planet is 90° west of the Sun.  If the superior planet is seen due south in the evening sky just after sunset, then the planet configuration is called eastern quadrature, since the planet is 90° east of the Sun.  Quadrature configurations are good times to view superior planets.  Through a telescope, a superior planet in quadrature appears slightly less than round or in its “gibbous” phase.

Planetary configurations aren’t typically noted by the public at large, but have been employed in almanacs for many decades.  Their use by the stargazer can alert one to the approximate position of the planet in its orbit.  Here’s how:  remember that most planetary configurations are expressed in terms of their alignment with the Sun.  That is, an inferior conjunction places a planet close to Earth; a superior conjunction places a planet farthest from Earth, that is, on the other side of the Sun.  A greatest elongation places an inferior planet at its best viewing relatively close to Earth either side of the Sun.  And opposition places a superior planet opposite the Sun in our sky, which means that the planet is best viewed at this time since it’s closest to Earth.

To make it easier to imagine these planetary configurations, you can visit the exedra in front of the Cleveland Museum of Natural History.  What’s the exedra?  It’s the circular portico in front of the conically shaped Shafran Planetarium that surrounds a representation of the planets closest to the Sun.  The exedra (with an emphasis on the first syllable) was designed to portray the positions of the planets at the 100th anniversary of the Museum’s opening, which will occur on December 13, 2020.

Since the exedra portrays the Sun, the orbits of the inferior planets Mercury and Venus, the orbit of Earth, and the superior planet Mars, you yourself can stand with family or friends along those orbits in the exedra to show planets in various conjunctions in relation to the Earth.  It may sound unusual to do this, but that very practice was used recently in one of our astronomy classes to convey the point of matching planet positions in their orbits with where to find planets in the sky.  And it worked.  Try it for yourself next time you visit us.  If you need help, ask our astronomers at the Museum.

There you have it.  Now you know how to find it when you ask:  So, where’s my planet?

Sky Roundup Almanac for Cleveland, June, 2007

June   1: Full Moon; Jupiter north of the Full Moon, well after sunset
June   8: Last Quarter Moon;
Venus at greatest elongation 45° east of the Sun (and brightly visible at sunset)
June 10: Mars south of waning Moon (and visible before dawn)
June 13: Venus just over ½° north of Beehive Cluster (M44), or 45° east of the Sun;
  Waning crescent Moon just less than 1° north of Pleiades
June 15: New Moon
June 18: Waxing crescent Moon just north of Beehive Cluster;
 Venus just over ½° south of Moon, but no occultation visible in Cleveland
June 19: Saturn less than ½° south of waxing crescent Moon, but no occultation visible;
Later, Regulus less than ½° south of Moon, but no occultation visible here;
Pluto at opposition (but a large telescope and detailed sky charts are needed)
June 21: Solstice, or the first day of Summer for Northern Hemisphere
June 22: First Quarter Moon
June 24: Spica just north of First Quarter Moon
June 28: Antares ½° north of waxing gibbous Moon, but no occultation visible here; Jupiter south of Moon
June 30: Full Moon; Venus and Saturn closing in on dazzling conjunction

Questions or comments? 
Please contact:  Roy Kaelin at 216.231.4600, x3405, or rkaelin@cmnh.org