18 June 2008
Planetary Roundup
Three major planets are well placed for viewing. This chart of Jupiter and the Moon and Sagittarius shows the changing view of the trio as the Moon moves across the sky the evenings of June 18 and 19. Jupiter and Sagittarius will remain in close proximity over the summer with Jupiter being the brightest beacon in the sky each night. Over the summer Jupiter will gradually become visible earlier and earlier in the evening. Saturn and Mars are slowly slipping into the evening glare but remain visible for a short while after sunset. Within the next month they will entirely fade from view, so catch a glimpse and enjoy the show of these two wanderers in the midst of Leo the Lion.
04 June 2008
The Southern Sky
As I write this entry, I am sitting in Melbourne, Australia on a business trip. Melbourne is approximately 38 degrees south of the equator and I have always likened this to San Francisco which is approximately 38 degrees north of the equator. I think of these two coastal cities having similar climate and similar sky conditions but in exactly the opposite way. Being 38 degrees north of the equator San Francisco is heading toward the summer solstice. The days are getting longer and the sky, as always, appears to pivot around the north star which is (by no coincidence) 38 degrees above the northern horizon, with the Sun and planets moving in a long arc across the southern half of the sky.
Melbourne presents a mirror image of the sky in many ways. The Sun does in fact still rise in the east and set in the west, but the Sun and planets move in a long arc across the *northern* part of the sky. The days here are short and getting shorter and Australia and other southern hemisphere countries prepare for the onset of winter in a few weeks. The mirror image of the sky in the northern hemisphere is visible in that the sky appears to pivot around the southern point of the sky. There is no bright star near the southern point of the sky so we don't have exactly the same scenario (ie. no south polar star like Polaris in the northern hemisphere), but the motions are similar -- in the opposite direction.
For example, in the northern hemisphere the Big Dipper moves around the north star in a large sweeping circle in a counter-clockwise direction, but in the southern hemisphere the southern constellations pivot around the south "pole" in a large *clockwise* circle every 24 hours. The Big Dipper and other star patterns and constellations near the north star are visible all night in the northern hemisphere and are called "circumpolar" stars, but are not at all visible from my location here in the southern hemisphere. Similarly, the circumpolar stars in the southern sky are not visible at all from San Francisco. Hence we never see the Southern Cross or Alpha Centauri (one of the nearest stars to Earth) or the Magellanic Clouds (nearby galaxies) from the northern hemisphere.
The early travelers setting sail across the seas and around the globe noted these changes in the sky and learned some valuable rules about celestial navigation. Although we don't depend upon these markers to tell us where we are on the globe anymore, I enjoy looking out for these changes in motion in the sky as I travel from time to time far north of San Francisco or in this case far south. The Southern Cross is a beautiful sight from here, even though the sky has been too overcast to see much else.
Melbourne presents a mirror image of the sky in many ways. The Sun does in fact still rise in the east and set in the west, but the Sun and planets move in a long arc across the *northern* part of the sky. The days here are short and getting shorter and Australia and other southern hemisphere countries prepare for the onset of winter in a few weeks. The mirror image of the sky in the northern hemisphere is visible in that the sky appears to pivot around the southern point of the sky. There is no bright star near the southern point of the sky so we don't have exactly the same scenario (ie. no south polar star like Polaris in the northern hemisphere), but the motions are similar -- in the opposite direction.
For example, in the northern hemisphere the Big Dipper moves around the north star in a large sweeping circle in a counter-clockwise direction, but in the southern hemisphere the southern constellations pivot around the south "pole" in a large *clockwise* circle every 24 hours. The Big Dipper and other star patterns and constellations near the north star are visible all night in the northern hemisphere and are called "circumpolar" stars, but are not at all visible from my location here in the southern hemisphere. Similarly, the circumpolar stars in the southern sky are not visible at all from San Francisco. Hence we never see the Southern Cross or Alpha Centauri (one of the nearest stars to Earth) or the Magellanic Clouds (nearby galaxies) from the northern hemisphere.
The early travelers setting sail across the seas and around the globe noted these changes in the sky and learned some valuable rules about celestial navigation. Although we don't depend upon these markers to tell us where we are on the globe anymore, I enjoy looking out for these changes in motion in the sky as I travel from time to time far north of San Francisco or in this case far south. The Southern Cross is a beautiful sight from here, even though the sky has been too overcast to see much else.
20 May 2008
19 May 2008
Toward the center of the Milky Way
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. 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.
When you look toward Sagittarius you are looking nearly at the center of the Milky Way. 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.
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!
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