24 February 2008

How to see Mercury

I've been asked in the past how to find Mercury. Because it is so close to the Sun compared to any other planet in the Solar System, it is harder to spot than the rest and never appears very far away from the Sun (that is, it never appears high up in the night sky). However, because it is relatively close to Earth, it can also become quite bright. These two factors combine to make Mercury a challenge to see but still a fun goal for many. The coming weeks offer an opportunity to try to see it.

Mercury and Venus are both circling the Sun inside the orbit of Earth and both move faster than Earth. In a single 12 month period of time, Mercury will circle the Sun over 4 times. As it speeds on its orbit, it is visible low on the horizon in the evening or morning sky. Right now both Mercury and Venus are very low on the horizon in the morning and if you have a clear view and good timing, you can see the two dance along the horizon. The diagram shows where to look for them.

In 2004 NASA launched the Messenger spacecraft to explore Mercury and in January 2008 it finally made its first pass, flying just over 100 miles above the surface of Mercury. Messenger is returning some amazing photographs of the planet. To my surprise, Mercury looks exceedingly similar to our own Moon. For pictures, visit http://www.nasa.gov/mission_pages/messenger/main/index.html.

Mercury and our Moon have much in common, being of similar age, made of similar minerals, and both having no atmosphere*, they are bombarded by meteors and have no erosion or surface flows which change its appearance over time. Hence they both appear similar on their surfaces. For me, seeing the photos reminds me that there are lots of things we share in common with our neighbors in space and by exploring our own Solar System we can better understand what is happening in other star systems and galaxies.

* Note: Mercury has an extremely thin atmosphere due to the solar wind lifting particles off the surface of the planet, but this layer of atoms is being constantly "burned off" and is not a permanent atmosphere as we have on Venus, Earth or Mars.

12 February 2008

Total Lunar Eclipse on February 20th

Mark your calendar for Wednesday February 20. Weather permitting, we will experience a total eclipse of the moon, one of the best timed eclipses in years and the last total lunar eclipse until December 2010. This one is timed well because it will be underway at sunset and will be at its peak during the 7:00 pm hour, much easier to see than the eclipse last August which took place in the middle of the night in San Francisco.

A total lunar eclipse is a beautiful showcase of many astronomical phenomena. Lunar eclipses were used by scientists hundreds of years ago to prove that the Earth is round because the darkening of the Moon is actually being caused by the curved shadow of the Earth. The early and late phases of a lunar eclipse are punctuated by the curved shadow of the Earth moving across the otherwise full Moon. These phases take a little over an hour to happen and as the Moon glides deeper into Earth's shadow, the illuminated surface of the Moon becomes less and less.

During the period of "totality" all direct sunlight is obscured by the Earth and the Moon fades into a dark disk, a rather stunning image to behold. Every total lunar eclipse has a different appearance because a small amount of sunlight is refracted by the atmosphere of the Earth and illuminates the otherwise-darkened surface of the Moon. The same effect that causes the sky to turn red, orange or pink at sunset is responsible for the color that you see on the Moon. Think of the light as the collective light of all the sunrises and sunsets around the entire globe. Since no direct sunlight is reaching the surface of the Moon during totality, anything we see will have been first refracted by the Earth's atmosphere, then it will have traveled 240,000 miles to the moon, and then will reflect back from the Moon to the Earth for us to enjoy!

If the Moon is very dark during the eclipse, you should be able to spot two bright objects on either side of the Moon. These are the bright star Regulus in the constellation Leo, and the planet Saturn. The entire grouping should fit nicely in the field of view of binoculars so get out whatever you have, dust it off, and pull up a chair. The totality phase of the eclipse will be from 7:00 until 7:52 pm.

By watching the eclipse from start to finish you can get a very good feel for just how far and fast the Moon moves in its orbit around the Earth. When the Sun sets on the evening of the 20th, the Moon will rise due east and quickly move across the southern half of the sky. This motion is due to the rotation of the Earth on its axis. However, the darkening of the Moon as it moves through the shadow of the Earth is due to the orbiting of the Moon around the Earth, something which is slower and thus more subtle. By watching the changing position of the Moon against Regulus and Saturn, you will be able to get a sense how far it moves against the background stars in a short span of time.

01 February 2008

Gorgeous Dawn Sky

Sometimes the stars do in fact align and all is well. OK, perhaps not the stars but the planets, and suddenly they shine in the sky as the clouds and rain in San Francisco have miraculously disappeared just in time. The morning of Friday February 1st is one of those special moments when the brightest objects in the sky line up and make for great viewing. As Venus graces the morning skyline, it is being joined by Jupiter which is rising each morning higher out of the morning glare of sunrise. They pass very near to each other on the morning of the 1st and are about a half degree apart - that's about one full moon's width. But the moon is not far off, a crescent just up and to the right next to the bright star Antares in Scorpio.

The next few weeks will offer the chance to see the change in the lineup, as Jupiter continues to climb each day in the morning sky while Venus stays low on the horizon. That is because Jupiter, being considerably further away from the earth, moves at more the same pace across the sky as the sun, whereas Venus, being much closer, actually is speeding around the Sun and stays lower on the horizon.