Year End Viewing

During 2008 I have found myself captivated by close groupings of the inner planets (Mercury and Venus) with other planets and the Moon. This is a fortunate thing because just about every month there is something to look forward to. The next few days will bring about this happy circumstance as Mercury makes an evening appearance swinging by Jupiter, and Venus grows in brilliance in the sunset sky as it moves through Capricorn. The diagram shows some beautiful configurations that you can see in the coming days. The Moon will be near Venus on the 30th and 31st, closing out 2008 high in the southwest after sunset.

Circumpolar Stars

Circumpolar stars are those stars that are nearest the north star Polaris. These stars are in a special category because they are always visible every night and rather than rise in the east and set in the west like most of the stars in the sky (and the moon and planets), instead they circumscribe Polaris every 24 hours, hence the designation circumpolar.

From our latitude in San Francisco, 38 degrees north of the equator, we see Polaris 38 degrees up above the horizon due north. Although it is a celebrated star because of its unique location, Polaris is not the brightest star in the sky - that honor goes to Sirius in Canis Major. However, being at the point in the sky where Earth's north pole projects into space means that Polaris does not move over the course of a day or even over the course of a year, a truly unique star!

The "circumpolar region" of the sky is a circle that stretches from Polaris 38 degrees down to the horizon and 38 degrees in every other direction around it. Everything that you can see in this zone remain above the horizon every night. All the stars and constellations move in a counter-clockwise direction around Polaris, much as you might imagine a pinwheel that is anchored to a center point but the body of the wheel can spin in a circle. The same is true of the constellations that are within that 38 degree circle.

Today I was at the San Francisco Waldorf School talking with the sixth grade class about Circumpolar Stars and many other topics of interest for the young astronomer. The students in this class had been creating a number of illustrations of star patterns including the zodiac and the circumpolar stars. The illustration is taken from the workbook of one of the students (Sophia) and is a fine illustration of the circumpolar stars as seen this time of year shortly after sunset. Polaris is the star at the end of the handle of the Little Dipper and the other constellations show are all within the 38 degree circle around Polaris. As mentioned, over the course of 24 hours these stars will all move in a very large circle in a counter-clockwise direction. Thus in the early evening Cassiopeia is near the top of its path looking like an "M" but over the course of the night it will swing around to the left of Polaris eventually moving down low on the horizon by morning. In contrast to this, the Big Dipper is low on the horizon at nightfall but by early morning will have swept around to the right of Polaris and will be high in the sky at sunrise.

For those wishing for a more advanced lesson on circumpolar stars, you'll be interested to know that as you move north from San Francisco, Polaris appears higher in the sky and the circumpolar region becomes larger. What happens if you move south toward the equator? It's a fun thing to ponder.

KFOG Broadcast: November 25th, 2008

Conjunction of Venus and Jupiter

Venus and Jupiter will have a conjunction at the beginning of the month of December. What is a conjunction? There are many uses of the term in astronomy, and here we use it to describe when two planets are very near to each other because they are on the same line of "longitude." I put the word longitude in quotes because in astronomy, we use a coordinate system similar to latitude and longitude, but the proper names are declination and right ascension. Therefore, when two planets have the same right ascension, we call that a conjunction.

The sketch (made by my daughter) depicts the sky as it will appear shortly after sunset on November 30th just as Jupiter and Venus are moving toward their closest approach. The scene will be made all the more beautiful by a young crescent moon just below the two planets. The next night, December 1st, the Moon will have moved just to the left and above the two planets. Both nights will be perfect for telescopic viewing, as the young Moon with its cratered surface (usually accompanied by earthshine) is always a beautiful sight, and you can see Venus at gibbous phase and Jupiter with its four bright moons. As mentioned in a previous post, these three objects with their special traits are three of Galileo's most famous discoveries with his telescope in the early seventeenth century.

Don't wait: Use your telescope now

This month offers a great opportunity to dig out that old telescope that you've rarely used and put it to work. Jupiter and Venus are putting on a show that will be of interest to anyone who has ever marveled at the heavens. These two planets are moving closer and closer and at the end of November will form a very compact grouping with the crescent Moon. What will you see?

Venus is brightening as it gradually moves closer to Earth in its orbit. Because it is nearer to the Sun than the Earth, it never climbs too high in the sky. For the rest of 2008 it is a brilliant evening "star", dominating the western horizon just after sunset. Jupiter, on the other hand, is much more distant from the Sun than the Earth and therefore over a year can be seen in all parts of the sky, moving gradually against the backdrop of stars but not nearly as quickly (from our Earth-bound perspective) as Venus. Jupiter has spent 2008 in the constellation Sagittarius and the diagram shows its present location, moving east of Sagittarius toward Capricorn.

Each evening for the next few weeks you will be able to see Jupiter and the background stars of Sagittarius gently drift toward the Sun while Venus holds its own. The combined effect is that Jupiter will move closer and closer to Venus. It is an experiment that requires nothing more than taking a moment each evening (or every couple days) to look west after sunset and note the relative position of these two bright objects. They are much brighter than anything else in the sky so you should not have any problem finding them. On November 17th Venus makes a very close encounter with one of the brighter stars in Sagittarius.

The evenings of November 30th and December 1st are particularly noteworthy as the young crescent Moon makes a close "fly-by" of the two planets, something that should be dazzling to see with a telescope or binoculars. Jupiter, of course, is impressive with its four bright moons, and Venus is always fascinating because it goes through phases just like our own Moon. By end of month, Venus will be a waning gibbous phase as it moves closer to Earth and slowly becomes a crescent over the next several weeks. Happy viewing!

Dark October Mornings

Last year the US Congress made a change in daylight savings time. Rather than end in October it now carries into November. The effect of this is that mornings are dark in October, making it hard to get up and get going. On the other hand, it makes it much easier to see the morning sky and in the coming days it's going to be noteworthy.

Sunrise is happening this week at 7:30 which means that up until 6:45 or so, the sky is dark enough to see stars and planets. The diagram shows where to look for the Moon, Saturn and even Mercury. In the Fall, the path of the planets and Moon (the ecliptic) is in a very steep line from the point of sunrise into the eastern sky. Hence Mercury will be visible just above the point where the Sun will be rising, and Saturn and the Moon in increasing distances above and to the south of Mercury.

I love to see the very old Moon in the last days of its cycle. The ever thinning crescent reflects more and more "earthshine" and glows like a jewel in the morning sky. As October comes to a close, the waning Moon on the 24th, 25th, 26th and 27th should be a striking sight as the sky begins to glow with the dawn.

Galileo discoveries redux

Nearly 400 years ago Galileo pointed a crude telescope toward the heavens and documented three remarkable things in the sky, discoveries that dramatically affected the understanding of the universe at that stage in history. With his telescope Galileo was able to see that the Moon was not a perfect circle, that Venus had phases, and that Jupiter had satellites of its own. Each of these posed a threat to the current body of knowledge of that era - Jupiter's moons and Venus' phases challenged the geocentric view of the universe and the non-perfect Moon challenged the Aristotelian theory of perfectly circular shapes in the universe.

With a simple telescope or even binoculars you can recreate some of these discoveries in the coming weeks and months. I'll write more about Venus in a future post. In October and November the phase of Venus will gradually transition from gibbous to half and later in the year to a crescent. The Moon is easy enough to study and is a marvel to view in any telescope or binoculars with the spectacular rocky edge of the surface always intriguing.

However, for the coming months Jupiter is the dominant "star" in the evening sky. Jupiter calls out for investigation because of the interesting texture of its surface and because of the changing position of its four largest moons. Called the Galilean satellites, these moons are visible to us even in low magnification and are interesting to observe because they change location so quickly -- even within the course of a few hours. They regularly pass through the shadow of Jupiter, yielding eclipses on a frequent basis.

From the nearest to the furthest moon, the names of the four satellites are Io, Europa, Ganymede and Callisto. Io is so close to Jupiter that it orbits the giant planet in less than two Earth days. Europa takes exactly twice as long and Ganymede twice as long again. Every few hours the overall pattern of these four moons is changed sufficiently to see new moons emerge from behind Jupiter, others disappear, and the overall pattern spread apart and then contract into a group.

Sky and Telescope Magazine has an excellent article on the moons and a very helpful pop-up screen that you can view in your web browser. It shows you the relative position of the moons of Jupiter at any time and can make a star gazing evening into a treasure hunt that will give you the feeling of discovery that will rival that of Galileo. So take the time to look to the south for the brightest object you can see (about halfway up the horizon above due south after sunset) and try it for yourself.

KFOG Broadcast: September 24th, 2008