As Saturn continues to dominate the evening sky, Jupiter now rules the morning sky. It shines brightly in the pre-dawn sky near the constellation Sagittarius. This week the Moon will pay a visit to the scene, passing through the "teapot" of Sagittarius on May 23rd and near Jupiter on May 24th.
When you look toward Sagittarius you are looking nearly at the center of the Milky Way Galaxy. While the Moon is passing through, you can't see much of the Milky Way at all, but once it has moved on and the light of the Moon doesn't impact the sky, you will have a better view of the center of our galaxy. Use binoculars and peer into Sagittarius and you will see a wealth of beauty, even with city lights. And if you don't want to get up so early in the morning for the view, wait until Summer and Sagittarius will be rising earlier and earlier in the night.
19 May 2008
13 May 2008
The Days Are Getting Longer
The days are getting longer as the Earth moves in its path around the Sun. Why do the days change in length? This has to do with the axis of the Earth being tipped. Why does that affect the length of a day?
For illustration, visualize of a globe mounted on a base. Globes are always mounted at an angle, representing the fact that the Earth is tipped 23.5 degrees from vertical. That is, as the Earth spins on its axis every 24 hours, the line around which it spins points up and we call that direction North. This point in the sky is fixed over a period of hundreds of years and changes very slowly such that in a lifetime, the axis of the Earth will point nearly perfectly toward a single point in the sky. It is pure coincidence that for us in the current era, this point in the sky is almost directly where the North Star can be found. This star, Polaris, is the current north star but in a few thousand years won't be. If you want to learn more about the changing location of the pole star, you will want to read this article on "precession of the equinoxes."
As the Earth travels on its 365-day journey around the Sun, the axis of the Earth remains fixed in the same direction, North. That means that for half of the year the north side of the Earth is tipped toward the Sun, and for the next half of the year the south side of the Earth is tipped toward the Sun. That leads to the seasonal change that we all come to understand. When the north side of the Earth is tipped toward the Sun, we get more hours of light each day in the northern hemisphere. We are in that situation right now and with each passing day between now and the Summer Solstice, the days become longer. A side effect of this is that the Sun appears to rise north of due east and set north of due west. As the Sun moves across the southern sky, it climbs higher with each successive day. In the San Francisco Bay Area, it will be nearly in the zenith on the day of the Summer Solstice. This diagram illustrates how the light changes on the Earth throughout the year. Enjoy the sunshine and warm weather and take note of how high the Sun climbs in the sky at midday.
Note: Mercury remains visible for a few more days and is changing phase rapidly. It is found in the north-west part of the sky just after sunset.
For illustration, visualize of a globe mounted on a base. Globes are always mounted at an angle, representing the fact that the Earth is tipped 23.5 degrees from vertical. That is, as the Earth spins on its axis every 24 hours, the line around which it spins points up and we call that direction North. This point in the sky is fixed over a period of hundreds of years and changes very slowly such that in a lifetime, the axis of the Earth will point nearly perfectly toward a single point in the sky. It is pure coincidence that for us in the current era, this point in the sky is almost directly where the North Star can be found. This star, Polaris, is the current north star but in a few thousand years won't be. If you want to learn more about the changing location of the pole star, you will want to read this article on "precession of the equinoxes."
As the Earth travels on its 365-day journey around the Sun, the axis of the Earth remains fixed in the same direction, North. That means that for half of the year the north side of the Earth is tipped toward the Sun, and for the next half of the year the south side of the Earth is tipped toward the Sun. That leads to the seasonal change that we all come to understand. When the north side of the Earth is tipped toward the Sun, we get more hours of light each day in the northern hemisphere. We are in that situation right now and with each passing day between now and the Summer Solstice, the days become longer. A side effect of this is that the Sun appears to rise north of due east and set north of due west. As the Sun moves across the southern sky, it climbs higher with each successive day. In the San Francisco Bay Area, it will be nearly in the zenith on the day of the Summer Solstice. This diagram illustrates how the light changes on the Earth throughout the year. Enjoy the sunshine and warm weather and take note of how high the Sun climbs in the sky at midday.
Note: Mercury remains visible for a few more days and is changing phase rapidly. It is found in the north-west part of the sky just after sunset.
29 April 2008
Planets in the Evening / The Winter Triangle in Spring
The western sky just after sunset is exploding with beauty and it is well worth the time to spend a few minutes soaking it in. Right now there are three planets on display in the evening sky along with the fine winter constellations still visible as the glow of dusk fades. I have been watching this over successive evenings and am impressed with the variety of colors and patterns visible after sunset. This week the sun drops below the horizon around 8:00 PM pacific daylight time and the sky is reasonably dark by 8:30, but by 9:00 the best viewing emerges.
First up is the fast-moving planet Mercury which will spend the next few weeks making a strong showing in the western sky. Because it is so close to the sun it never moves far above the horizon, but we will be seeing it at one of the best views of the year. This diagram shows Mercury in a close encounter with the Pleiades on May 1st, but it will continue to hover in the west through mid-month. It is bright enough to be seen in the glare of the dusk sky and you will see it best if you have a clear western horizon.
Next is Mars which continues to share the limelight with the twin stars of Gemini, Castor and Pollux. Mars is distinctive in comparison to the Twins because of its deep orange color. It is slowly diminishing in brightness but is still bright enough to be easy to find.
Saturn is the highlight of the planetary lineup, high in the sky after sunset in the constellation of Leo. More on Saturn can be found in my last post.
The bright constellation Orion is now setting in the glare of sunset, slowly fading from view as the constellations of summer arrive in the east and begin their reign. Nonetheless, a very bright grouping is still visible in the western sky featuring three stars from three different constellations. A distinctive grouping of stars from different constellations is known as an "asterism" and the Winter Triangle is one of the more distinctive asterisms in the sky. It contains Sirius, the brightest star in the sky which is in the constellation Canis Major, Procyon, the brightest star in Canis Minor, and Betelgeuse, which is in the constellation Orion. This diagram provides a helpful guide to this pattern. Each of these stars have distinctive colors, with Betelgeuse an orange-red (it is a red giant star), Sirius whitish-blue, and Procyon in-between the two as a plain white star. Enjoy the sky this spring - it's full of remarkable things to see.
First up is the fast-moving planet Mercury which will spend the next few weeks making a strong showing in the western sky. Because it is so close to the sun it never moves far above the horizon, but we will be seeing it at one of the best views of the year. This diagram shows Mercury in a close encounter with the Pleiades on May 1st, but it will continue to hover in the west through mid-month. It is bright enough to be seen in the glare of the dusk sky and you will see it best if you have a clear western horizon.
Next is Mars which continues to share the limelight with the twin stars of Gemini, Castor and Pollux. Mars is distinctive in comparison to the Twins because of its deep orange color. It is slowly diminishing in brightness but is still bright enough to be easy to find.
Saturn is the highlight of the planetary lineup, high in the sky after sunset in the constellation of Leo. More on Saturn can be found in my last post.
The bright constellation Orion is now setting in the glare of sunset, slowly fading from view as the constellations of summer arrive in the east and begin their reign. Nonetheless, a very bright grouping is still visible in the western sky featuring three stars from three different constellations. A distinctive grouping of stars from different constellations is known as an "asterism" and the Winter Triangle is one of the more distinctive asterisms in the sky. It contains Sirius, the brightest star in the sky which is in the constellation Canis Major, Procyon, the brightest star in Canis Minor, and Betelgeuse, which is in the constellation Orion. This diagram provides a helpful guide to this pattern. Each of these stars have distinctive colors, with Betelgeuse an orange-red (it is a red giant star), Sirius whitish-blue, and Procyon in-between the two as a plain white star. Enjoy the sky this spring - it's full of remarkable things to see.
09 April 2008
It's Spring: See Saturn and Mars
This Spring there are two bright planets in the evening sky that you should stop and see when the weather permits. The first is Saturn, the second is Mars.
Saturn is well placed for viewing in the evening and is high in the sky after sunset. Saturn is currently spending several months in the constellation Leo and Saturn itself is near the bright bluish star Regulus which is the brightest star in Leo. They are nearly directly overhead in the evening, slightly to the south. Saturn and its famous ring system are very large compared to all the other planets and therefore reflect a lot of sunlight back to Earth. Saturn is a fairly colorless planet despite what you see in photographs which are often color enhanced. In natural light, Saturn is a blend of off-white, yellow and grey hues and therefore from our point of view looks like a medium-bright yellow-white star.
As Saturn travels in its 29 year path around the Sun, the ring system is more tilted toward Earth for a period of time and then less tilted toward Earth at other times. Sometimes we see the ring system edge-on and because it is so thin, it is very difficult to see the rings at all. For this reason, Saturn is sometime brighter and sometimes dimmer. Right now it is going through a brighter phase and with binoculars stands out as something other than a star. Find a comfortable spot (or lie on a blanket) and spend a few moments looking up near the highest point in the sky. In binoculars, Saturn looks like an oval, brighter than the surrounding stars and shaped differently than everything around it.
Mars is also well placed for viewing in the evening and is in the western sky after sunset. Mars is currently spending time in the constellation Gemini near the twin stars of Castor and Pollux. This diagram shows where to locate it later this week when the Moon passes near it on April 11th. Mars is one of the closest neighbors to Earth and when the alignment is right, can be quite bright. However, at this point it and the Earth are moving further apart and as such, it's brightness is fading from week to week. It still stands out among the stars, and of course it is a distinctive color of orange so it is easy to spot. But unless you know where to look, you might not realize it is a planet and instead think it is just another star. So use the Moon as your guide this week and take it in.
Happy viewing!
Saturn is well placed for viewing in the evening and is high in the sky after sunset. Saturn is currently spending several months in the constellation Leo and Saturn itself is near the bright bluish star Regulus which is the brightest star in Leo. They are nearly directly overhead in the evening, slightly to the south. Saturn and its famous ring system are very large compared to all the other planets and therefore reflect a lot of sunlight back to Earth. Saturn is a fairly colorless planet despite what you see in photographs which are often color enhanced. In natural light, Saturn is a blend of off-white, yellow and grey hues and therefore from our point of view looks like a medium-bright yellow-white star.
As Saturn travels in its 29 year path around the Sun, the ring system is more tilted toward Earth for a period of time and then less tilted toward Earth at other times. Sometimes we see the ring system edge-on and because it is so thin, it is very difficult to see the rings at all. For this reason, Saturn is sometime brighter and sometimes dimmer. Right now it is going through a brighter phase and with binoculars stands out as something other than a star. Find a comfortable spot (or lie on a blanket) and spend a few moments looking up near the highest point in the sky. In binoculars, Saturn looks like an oval, brighter than the surrounding stars and shaped differently than everything around it.
Mars is also well placed for viewing in the evening and is in the western sky after sunset. Mars is currently spending time in the constellation Gemini near the twin stars of Castor and Pollux. This diagram shows where to locate it later this week when the Moon passes near it on April 11th. Mars is one of the closest neighbors to Earth and when the alignment is right, can be quite bright. However, at this point it and the Earth are moving further apart and as such, it's brightness is fading from week to week. It still stands out among the stars, and of course it is a distinctive color of orange so it is easy to spot. But unless you know where to look, you might not realize it is a planet and instead think it is just another star. So use the Moon as your guide this week and take it in.
Happy viewing!
11 March 2008
Space Shuttle and Space Station visibility
The Space Shuttle launched this morning and will be orbiting Earth for the next few weeks. While orbiting, it will be docked with the International Space Station from the 12th to the 24th. The two will be visible on a regular basis for brief periods just before sunrise or just after sunset. Look here for details of visibility from San Francisco. You can also check visibility for any location on Earth.
04 March 2008
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.
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.
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.
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