Seeing Mercury this week
Published on Sep 30, 2007 at 12:21 pm.
1 Comment.
Filed under astronomy, skywatching.
Mercury swings around the Sun very quickly. It takes only 88 days to make a complete orbit. Being quite a bit closer to the Sun than the Earth, Mercury never seems to stray far from the Sun in the sky. However, it is now about 25° to the east of the Sun in the sky. It actually reached its farthest from the Sun yesterday (26°). It will be well away from the Sun for about the next week. So, Mercury is far from the Sun. That means that we should go look at it, right? Well, not quite so fast. Mercury may be far from the Sun, but whether you see it well depends upon where you live.
The Earth and the rest of the planets orbit the Sun almost in a plane. The plane described by Earth’s orbit is called the ecliptic. From Earth’s vantage point, the Sun always appears to be along the ecliptic. Since the other planets have orbits in a similar plane, they also always appear to be near the ecliptic in the sky. So, Mercury generally does not appear due east or due west of the Sun in the sky as it moves along its orbit, but rather will appear to be eastward or westward of the Sun along the ecliptic. Right now Mercury will appear about 25° from the Sun eastward along the ecliptic.
But, Earth’s rotational axis is not perpendicular to its orbit. Instead, Earth’s axis tilts about 23.5° from perpendicular. The effect of that tilt is that sometimes the Sun is more nearly overhead in the southern hemisphere, and sometimes more nearly overhead in the northern hemisphere. This is the reason for the seasons.
Just last week, Earth passed the point where the Sun was most nearly over the equator. It had been shining more on the northern hemisphere. Now, for about the next six months, it will be shining more on the southern hemisphere. The Sun will appear to be a bit farther to the south in the sky each day until the solstice on December 22, 2007. Each successive day the Sun appears almost a degree eastward in the sky relative to the fixed stars due to Earth’s motion around its orbit. Since it is also getting farther south in the sky, the ecliptic at this time of year is tilted towards the South. For those of us in the northern hemisphere, the whole celestial sphere appears tilted to the south, so the ecliptic appears to be tilted even farther to the south. This means that the a distance of 25° from the Sun along the ecliptic does not mean a difference of 25° in altitude above the horizon for Mercury. So, Mercury will still appear very low in the sky, as seen in my sky view above (If the image is too small to see clearly, click on it to get a much larger view). This image is created for Texas, but the view will be similar for anywhere in the northern temperate latitudes. Mercury will appear lower in the sky the higher your latitude, and higher in the sky the closer you are to the equator.
But, this is the view from the Northern Hemisphere. Quite the reverse situation exists for my readers south of the equator. From the Southern Hemisphere, the celestial sphere appears tilted towards the north. So, the ecliptic being tilted towards the south partially offsets that effect. The result is that the ecliptic appears to stand up almost vertical if you are at the right southern latitudes. I created the following sky view for the latitude of Sydney, Australia, but the view will be the about the same for anywhere within ten or fifteen degrees of that latitude (in other words, most all of the southern temperate latitudes). Now, you see that with the ecliptic tilted almost straight up, a change of 25° along the ecliptic corresponds almost to a change of 25° altitude with respect to the horizon.
So, what does all of this mean for people who want to go see Mercury? Well, it means that this is a very favorable elongation of Mercury for observers in the Southern Hemisphere and a very unfavorable elongation for observers in the Northern Hemisphere (The term elongation refers to how far the planet appears from the Sun.). For observers in the Northern Hemisphere, you may have a change of seeing Mercury in binoculars right after sunset during evening twilight. However, if you go looking for Mercury BE CAREFUL not to look at the Sun. Wait until the Sun is below the horizon before attempting to find Mercury. The planet Mercury will appear as a “star” up and to the left of where the Sun set. As a guide, try to find the planet Jupiter, which will be a very bright object in the southwestern sky. Since all the planets appear to move in similar orbits, and all appear close to the ecliptic in the sky, then the Sun, Mercury, and Jupiter will all appear to just about form a line in the sky. Draw an imaginary line between the Sun and Jupiter. Mercury is close to 1/3 of the way from the Sun to Jupiter along that line. Don’t wait too late after sunset, though, because Mercury will set not long after the Sun, even while it is still twilight. For Southern Hemisphere observers, it is much easier. Just note the location of Sunset on the horizon. As it twilight gets darker, you’ll see a fairly bright object just to the south of the line betwee Jupiter and where the Sun set. That will be the planet Mercury.
So, if you go looking for Mercury, good luck. Don’t be too disappointed if you don’t see it from the Northern Hemisphere. It is tough. There will be other times when Mercury will be much easier to see. But, there should be no problem seeing Mercury from the Southern Hemisphere. And, those of you in the Southern Hemisphere may want to go look because this will be about the best view of Mercury for quite a while.
-Astroprof
Sky views created using Stellarium
A Ler…-- Rastos de Luz on October 1, 2007 at 3:47 pm: 1
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