20 December 2006

Winter Solstice

Winter officially arrives Thursday December 21st at 4:22 pm. This is the quiet conclusion of six months of change as the Earth moves from the first point of Summer (Summer Solstice) and travels approximately 300 million miles around the Sun to the first point of Winter (Winter Solstice). On Thursday, the exact time from sunset to sunrise in San Francisco is 14 hours and 27 minutes - the longest night of the year. In the southern hemisphere of the Earth this is reversed and Thursday marks the longest day and the start of Summer. There is a lovely symmetry in the ebb and flow of the seasons. The Winter Solstice is a very important day in the ever-flowing cycles of the Sun and Earth because although the Solstice signals the completion of our move into darkness, there is hope for the future because the solstice indicates that the days are now going to be getting longer.

The skies have been changing and the cloud cover as well. If you love to look at the sky and see the stars, moon and planets, then you have been unhappy with the number of cloudy nights in December - that is, until this last week when the clear skies afforded a dazzling view of the winter sky. If the coming days allow, there will be fantastic sunset views of the Moon and Venus as they emerge from the glare of the Sun and dominate the western sky. The Moon, of course, moves quickly through its phases and as it does so, it appears higher and higher in the sky at the moment of sunset. By watching the sky from the same place on consecutive nights you can detect the motion and see the changing phase of the Moon. On the other hand, Venus is rising out of the glare of the Sun and will, over the next six months, brighten and move higher in the sky.

15 November 2006

Leonid Meteor Shower

Meteor showers are some of the most exciting spectacles to watch in all of astronomy. However, the best views require dark, clear eastern skies and a willingness to be awake when most people are sleeping. And this time of year, they require warm clothes! If skies are clear, the Leonid Meteor Shower should be visible this weekend during the morning hours of November 17-18 and 18-19.

Meteors are tiny bits of rock and dust that enter the earth's atmosphere and burn up. These bits of rock and dust are floating in long orbits in space and the earth "runs into" these clouds of rock and dust. Because the earth is moving so fast, the rocks and dust that are struck by the earth heat up from the friction of earth's atmosphere. The result of this are brilliant streams of light that are often called "shooting stars" but aren't stars at all, just very small visitors that shine briefly and flicker out.

Most major meteor showers repeat on an annual basis. Why is that? The bits of dust and rock that cause meteor showers are typically the remnants of a comet or other object in the Solar System which moved across the sky tens, hundreds or even thousands of years ago. The debris trail marks the path that the comet took some time ago. If that path intersects the earth's orbit then we experience a meteor shower each time the earth passes that specific point in space. Many people are familiar with a summer meteor shower that takes place around August 11-12. This shower is called the Persied Meteor Shower and is one of the best of the year.

The meteor shower coming up this weekend is called the Leonids and it is the result of a comet known as Temple-Tuttle (comets are usually named after their discoverers). The shower is called Leonids because the comets appear to originate in the part of the sky where we find the constellation Leo the Lion. This constellation does not rise in the east until very late in the night and as such, we don't get a good view of the Leonids meteors until after midnight. You won't see all of them at once but rather one every few minutes if conditions are good. Dawn does not break until 6:00 am each day this weekend so I will be looking out at 5:30 or so instead of staying up until 2:00 or 3:00 in the morning. How about you?

18 October 2006


October is a fascinating month for star gazing. A very special thing happens because the nights are getting longer and the sun doesn't rise until after 7:00 am throughout the month, so the view of the sky in the morning is quite nice and is worth an extra look this week. The diagram shows that the waning crescent moon is creating some beautiful patterns in the morning sky. Looking due east from 6:00 to 7:15 am you can see the bright star Regulus and the planet Saturn dominate the sky. The waning moon is just below this pair and is quickly getting thinner and lower on the mornings of October 18th and 19th. It is always a challenge to see a thin crescent moon but it is worth finding a clear eastern horizon because a thin crescent moon always has a deep glow of "earthshine" glowing on it.

Earthshine is the light of the earth reflected onto the moon. It creates a beautiful glow on the crescent moon and illuminates the dark portion quite well. Think of Earthshine as you would the bright glow around you at night under a full moon. Well, when the moon is a thin crescent the moon is experiencing a "full earth" which makes the dark moon surface glow an eerie grey-white color.

Fall mornings also offer another special reward: you are getting a sneak-preview of the winter sky. The sky at 6:00 am is the same sky you will see in the evening in winter. This includes favorite constellations such as Orion, bright stars such as Sirius, and bright clusters such as the Pleiades. I enjoy these sights when I step outside early in the morning to get the newspaper. I can't help but walk out and gaze at the splendor of the winter sky. Take a few moments some time soon and see it for yourself.

13 September 2006

Summer Zodiac / Pluto

As we approach the Autumnal Equinox the days are rapidly getting shorter. Most people can sense the change because of the earlier onset of darkness. Today sunset is at 7:20 pm and a short while later you can spot Jupiter not far from the point of sunset in the west. In one week sunset will be ten minutes earlier. The first official day of Fall is September 22nd, the date when the Sun rises due east and sets due west.

From a south-west facing view the evening sky is very impressive. As mentioned, Jupiter is visible not long after sunset and by 8:00 pm other stars begin to appear. If you have a clear south-west view you can enjoy a few late-Summer celestial wonders. Jupiter is by far the brightest object in the evening sky and is easy to spot low in the south-west after sunset. Looking further south (to the left) from Jupiter you will spot a bright orange-colored star. This is Antares, a red giant star at the heart of the constellation Scorpius. The constellation is quite large and has a number of bright stars. Just to the left of Scorpius is Sagittarius, something I visualize as a big teapot in the sky. These constellations are low in the south in Fall and will soon disappear into the sunset glare as the Sun "moves" into these constellations. That is why people born in November and December have these zodiac signs.

During the summer you might have heard the news that Pluto has been demoted from "planet" status to "dwarf planet" status. Although this is sad news for most everyone, I am not so troubled. By taking Pluto out of the planetary line-up, the Solar System is now made up of 8 planets which group nicely into two groups: (a) the 4 inner planets which are smaller in size and have rocky surfaces, and (b) the 4 outer giant planets which are made up of gas rather than having solid surfaces. Also, Pluto is so far it is extremely difficult to see from earth even with a large telescope. I have never even tried to locate it on my own because it is so dim and small it would not look like anything more than a faint speck of light. Nonetheless, it will remain important because it played a role in our understanding of the Solar System and I think it will still be celebrated at museums and planetariums for years to come.

02 June 2006

Summer Evenings / Beehive Cluster

Summer arrives on June 21st this year as the Sun moves to its northernmost point in the sky for the year. The total time from sunrise to sunset is 14 hours and 45 minutes! Throughout June and July we have to be patient and wait until well past 9:00 pm to enjoy dark skies. But June promises to be a great time for skygazers as we will have excellent evening views of four bright planets. Saturn and Mars are prominent in the sunset sky. Mercury starts the month very low on the western horizon but moves up slowly each evening and will be more readily visible as the month progresses. Jupiter is the brightest object in the sky in June and can be seen in the south-east during the evening hours.

Mars, being much closer to Earth than Saturn, appears to move more rapidly across the sky than Saturn. Mars is a short distance "below" Saturn at the end of May. By mid-June, however, Mars will have moved along the ecliptic and will be extremely close to Saturn, about a full-moon distance between them. During this time, Saturn and Mars are also very close to a well-known star cluster known as the Beehive Cluster. By mid-June, with 7x or 10x binoculars, you should be able to capture Mars, Saturn and the Beehive Cluster in one field of view.

The Beehive Cluster is one of 110 special objects in the night sky that were classified into a famous catalog by the French astronomer Charles Messier over 200 years ago. The Messier Catalog includes many of the most beautiful deep space objects such as nebulae, clusters and galaxies. The Beehive Cluster is known as M44 in the catalog, and during the Summer some of the most lovely Messier objects are visible in the rich Milky Way regions of Scorpio and Sagittarius. The on-line encyclopedia "Wikipedia" has an outstanding collection of images and descriptions of all 110 objects in the Messier Catalog. I recommend a visit if you want to see more. Go to www.wikipedia.org and search for "List of Messier Objects."

10 May 2006

Jupiter and the Full Moon

As Summer approaches, there are three major planets visible in the night sky. Saturn continues to glow high in the western sky after sunset near the twin stars of Gemini. Mars is lower in the west, a slowly fading red "star" visible after sunset. But the new giant of the evening sky is Jupiter which is just arriving in the east after sunset and will remain a blazing bright object in the sky for the Summer.

This month Jupiter is visible in the south-east sky. It is at "opposition" which means that the Sun, Earth and Jupiter are all in a straight line in early May. For that reason Jupiter rises at sunset, reaches its highest point in the sky at midnight, and sets at sunrise. Additionally it is closest to the earth and is therefore brightest and looks the largest when viewed through a telescope.

The full Moon passes very close to Jupiter on the nights of May 11 and May 12. Given Jupiter's intense brightness, it will still be very distinct next to the full Moon. You can notice on these nights that stars near the Moon will nearly fade from view but the brightness of Jupiter is so strong that it remains easy to spot. The one star that will remain somewhat visible is Spica, the brightest star in the constellation Virgo. On the diagram you can see that the Moon will be very near Spica on the evening of May 10.

If you have a small telescope or binoculars it's worth viewing Jupiter because you can see the four largest moons of Jupiter near the planet. Through 7x or 10x binoculars they will be small points of light in a straight line near Jupiter. Through a telescope they are quite distinct and line up beautifully. These four moons are known as the Galilean moons because they were first observed and documented by Galileo.

06 April 2006

Ecliptic / Solar Eclipse

From our earthbound perspective we see the Sun, Moon and planets all move across the heavens in a small band known as the Ecliptic. In this same band we find the 12 signs of the Zodiac -- that is, 12 familiar constellations through which the Sun, Moon and planets wander during the 12 months of the year. This band stretches from east to west through the southern part of the sky. Right now in the springtime the band is quite high in the evening sky. The attached illustration shows the position of the Moon and Saturn this evening (April 6th) where they are visible just next to the "twin" stars in Gemini. The Moon moves once around the heavens every 29 days so each day you can easily notice its daily change in position. Saturn, on the other hand, is so far away it takes years to move once around the Sun. Consequently you can find Saturn near Gemini each night for the forseeable future. If you watch the Moon change position each night and look for Saturn and Jupiter and the other planets you will come to know the Ecliptic quite easily.

Here's a brief report from Turkey where my family and I observed the total solar eclipse on March 29th. We had clear skies and an excellent view from the Mediterranean coast of Turkey near the ancient city Side. Brittany (grade 6) describes the scene as follows. "The horizon looked like sunset all around but the Sun was high above, covered by the Moon. The Sun's corona looked really interesting, like flames shooting out of the Sun. It went from being a hot day to a cold one during the eclipse. My brother and I had just been swimming but it was so cold we had to get dressed again."

A total solar eclipse is an impressive sight indeed and there are many websites and magazines with lovely pictures of last week's eclipse. But a camera cannot replace the experience of standing in the shadow of the Moon! If you get a chance to see one, you will not regret the effort it took to get there. If you want to see one in the USA the next total solar eclipse will be in 2017. If you are willing to travel outside the USA, you can see a total solar eclipse in August 2008 or July 2009.

22 March 2006


Eclipses of the Sun and Moon are special, rare events where we get to witness and take part in motions of the Solar System.

Why do we have eclipses? The path of the Sun across the sky is called the Ecliptic. The path of the Moon follows the Sun very closely but not exactly -- that is, the Moon follows the Ecliptic but is sometimes above and sometimes below it. However, during special moments every six months the path of Moon exactly intersects the Ecliptic and the result is that we experience a Lunar Eclipse at full moon (in which the Moon is darkened as it passes through the shadow of the Earth) or a Solar Eclipse at new moon (in which a part of the Earth is darkened as it passes through the shadow of the Moon), or both in a two week period.

On Wednesday March 29th the Moon will block the Sun and the shadow of the Moon will land on a broad swath of Earth from western Africa to central Asia. In this area people will observe an eclipse of the Sun. Along a narrow strip of Earth in the middle of this region the dark central shadow of the Moon (known as the umbral shadow) will create the stunning visual impact of a total eclipse of the Sun. During a total eclipse of the Sun the viewer on Earth witnesses the Moon completely covering the solar disk. This causes the sky to darken to nearly the condition of night, the temperature to drop and for the air to become still. Animals change their behavior in response to this sudden and dramatic change in the sky. For a few brief minutes in the total phase of the eclipse it is safe to look directly at the Sun because the central disk (photosphere) is covered. At this moment the faint outer regions of the Sun (corona) becomes visible. It is an awesome and beautiful experience.

Unfortunately, total solar eclipses happen infrequently and in nearly all cases you will need to travel to see one. The next total solar eclipse after March 29th won't take place until August 2008 and you will need to travel to Russia or China to see it. If you want to see one in North America you will need to wait until August 2017 and even then you will need to travel to another state to see it since it will only be visible in a line stretching from Oregon to South Carolina.

For more details on eclipses Nasa sponsors an excellent website: http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

01 March 2006

Vernal Equinox

Spring is in the air. Well, you might not realize that from the cold and the rain. But in fact you can tell by the changes in the sky. In particular, the days are getting longer and if you hadn't yet noticed, the pace of change in the amount of daylight is rapid.

The Vernal Equinox (the first day of Spring) takes place this year on March 20th. This is the day when the Sun stands directly over the Earth's equator at noon and begins its six month journey in the northern skies. From our perspective the Sun will be 52 degrees above the horizon at noon (which by the way is determined by subtracting our latitude in San Francisco from 90 degrees - a bit tricky to explain in writing so if you want to know more about this come to my lecture on March 21st!).

What does this mean for us right now? Each day the time of sunrise is arriving about 1 1/2 minutes earlier and the time of sunset is occurring about 1 minute later than the day before. So in a single week in March we experience more than 15 minutes of additional daylight compared to the week before. The Vernal Equinox is the day in which this daily change is fastest -- that is, the days are getting longer at the most rapid rate. After the Vernal Equinox the days will continue to get longer but the daily change will be slower.

All of this happens because of the tilt of the Earth on its axis. The tilt of the Earth gives us longer and shorter days and hence warmer and colder seasons. And it means that the location of the Sun, Moon and planets is not a simple, predictable line across the sky. We will look at some of the special motions of our Solar System neighbors in a coming issue about eclipses.