Solar System & Eclipses
As August approaches each year, we start to receive emails that claim Mars is approaching very close to the Earth, and that it will appear "as large as the full moon" in the night sky. A sample of these typical emails follows - complete with spelling errors.
Hoax: Planet Mars will be the brightest in the night sky starting August. It will look as large as the full moon to the naked eye. This will cultimate on Aug. 27 when Mars comes within 34.65M miles of Earth. Be sure to watch the sky on Aug. 27 12:30 am. It will look like The Earth has 2 Moons. Don't Miss it..... The next time Mars may come this close is in 2287. NOTE: Share this with ur friends as NO ONE ALIVE TODAY will ever see it again.
Hoax: Mars the Red Planet is about to be spectacular! This month and next, Earth is catching up with Mars in an encounter that will culminate in the closest approach between the two planets in recorded history. The next time Mars may come this close is in 2287. Due to the way Jupiter's gravity tugs on Mars and perturbs its orbit, astronomers can only be certain that Mars has not come this close to Earth in the Last 5,000 years, but it may be as long as 60,000 years before it happens again. The encounter will culminate on August 27th when Mars comes to within 34,649,589 miles of Earth and will be (next to the moon) the brightest object in the night sky. It will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification Mars will look as large as the full moon to the naked eye . Mars will be easy to spot. At the beginning of August it will rise in the east at 10p.m. and reach its azimuth at about 3 a.m. By the end of August when the two planets are closest, Mars will rise at nightfall and reach its highest point in the sky at 12:30a.m. That's pretty convenient to see something that no human being has seen in recorded history. So, mark your calendar at the beginning of August to see Mars grow progressively brighter and brighter throughout the month. Share this with your children and grandchildren. NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN.
Usually these emails come with images of Mars and the Moon - side by side appearing the same size.
Strangely enough, they are variations of original authentic emails generated in 2003 when Mars did in fact make one of its closest approaches to Earth in a very long time. But even then, Mars was still a long long way from Earth. In August 2003, if you observed Mars through a good telescope using at least 75x magnification - then Mars could look the same apparent size (in the telescope's eyepiece view) as the full Moon could look to the naked eye view.
This information has been doctored and recycled each August since 2003 and is now completely out of date.
The Facts
Some basic factual information about Mars are as follows:
- Mars could only appear as "large as the full moon to the naked eye" if you were in a spacecraft approaching Mars - and very close!
- The size (angular diameter) it takes in the sky is a tiny 8 seconds of arc. There are 60 seconds of arc in 1 minute of arc - and there are 60 minutes of arc in 1 degree. In comparison the (full) moon is about 1/2 degree or 1,800 seconds of arc (Mars is 8 seconds of arc or 225 times smaller).
- Earth 'catches up' to Mars in their respective orbits roughly every 2 years - and that is when they are closest to each other - referred to as opposition. Because both orbits are ellipses, and not circular, the distance between the two planets varies each time they are closest to each other. At closest they are about 57 million kilometres - at farthest just less than 100 million kilometres. But each encounter is (at the moment) becoming further away, and then gradually they will be closer again at each encounter. This is a 15 year cyclic phenomena that has been happening for... well since the solar system has been in its current configuration - billions of years. BUT - never will the two planets be so close that Mars "looks as big as the moon". At the absolute closest, Mars will still just look like a (reasonably) bright pinkish star to the naked eye.
- Unless you are going to be dead before within the next two years... you will see Mars at a 'close' approach again! And the best views will always be through a quality telescope.
There are quite a few web sites that discuss the "Mars is Big" hoax.
In 2003, Mars made headlines and as it reached opposition. Thousands of people attended viewing nights around the nation to get a glimpse at the planet, which has captured the curiosity of mankind since the dawn of age. Seeing Mars through the 20-inch at Stockport Observatory was certainly memorable as it would occasionally 'snap' into focus, revealing a tremendous amount of surface detail that one could only dream of.
Historically, the opposition of 2003 was so special because Mars would not come so close to our own until the year 2287 -- quite a wait even for the youngest of us!
Oppositions occur every 26 months as Earth catches up to the red planet, passing relatively close to it. During opposition, Mars and the Sun are on directly opposite sides of Earth. Because the two planets are so close, it's the best opportunity to observe Mars. But since our orbits are elliptical and there is a slight 'tilt', the close approach to Earth actually occurrs on 14 April (apparition). However, the difference in the apparent diameter will be negligible and will be favorable for the rest of the month.
The polar caps should be clearly visible through even the smallest of telescopes, whilst larger instruments will reveal some of the more subtle surface detail. Of course, this assumes that the weather is good on both our planets! Dust storms on Mars can quickly engulf the entire planet, dulling its surface.
If you have trouble seeing anything but an orange, featureless disc, then there are a things you should check.
Are the optics of your telescope good quality? Bad optics will never focus sharply and image contrast will always be poor. In a reflecting telescope if the mirror is homemade, has it been tested so that you know if it can perform well?
Collimation, (optical alignment) could be the problem.
Bad seeing (turbulence). If Mars is low in the sky, atmospheric turbulence can obliterate contrast and detail on the planet. Wait for it to gain altitude, say greater than 30 degrees. Sometimes, seeing may be bad even when Mars is high. It depends on atmospheric conditions at the time.
Speaking of turbulence, thermal currents inside the telescope tube will have a similar effect if the telescope is transported from a warm interior to cold outdoors. Allow the scope 1/2 to 1 hour to reach thermal equilibrium outdoors before beginning to search for planetary detail.
Even in an 8-inch telescope with good optics and good seeing conditions, the dusky markings on the planet require some patience and practice to discern especially if you are new to observing. The bright polar cap however should be easily visible.
In a 2.4-inch telescope the most prominent markings, Syrtis Major and the polar cap are visible. In larger apertures Mare Sirenum, Sinus Sabius, Mare Cimmerium and the dark polar hood surrounding the polar cap are visible.
Sometimes a larger aperture performs worse than a smaller aperture under bad seeing conditions. You may have been caught out by this. It depends on the prevailing general seeing conditions in your locality which may be determined by season and the thermal effects of adjacent buildings, asphalt, bodies of water, vegetation and other such factors. You might like to try a 4-inch or 6-inch aperture mask over the top end of the scope to see if this beats the seeing conditions. It might also cure a badly figured homemade mirror if that is the problem.
A red or orange filter will help if the seeing is good to begin with. Otherwise it makes little difference. On a recent occasion an orange filter improved the contrast on Mars in a 31/2-inch Maksutov telescope but made no difference in a 6-inch Newtonian. A filter however, even if it does nothing else, can reduce the 'glariness' of the planet so that it is easier on the eye.
Provided the optics in your telescope are good to begin with, keep on persisting with the above points in mind and you are sure to get a good night when Mars will deliver! As a rule, with good optics, under good seeing conditions, a larger aperture will always outperform a smaller one.
What is a Solar Eclipse?
A solar eclipse occurs when the moon crosses the path between the sun and the earth. If the motions of the celestial bodies were simple, there would be a solar eclipse every month. However, the sun and the moon have complicated motions, which make it difficult for the path of the moon to intersect the path between the sun and the earth. Despite this complication, eclipses are more common than people tend to believe.
The darkest shadow (where the sun is completely covered) is called the umbra. The umbra is narrow at the distance of the Earth, and a total eclipse is observable only within the narrow strip of land or sea over which the umbra passes. The partial shadow is called the penumbra. A partial eclipse may be seen from places within the large area covered by the penumbra. Sometimes the Earth intercepts the penumbra of the Moon but is missed by its umbra in this case a partial eclipse of the Sun is observed. The sun is so bright that even though it is partially covered it can illuminate the Earth.
Nature has created a remarkable coincidence, the sizes and distances of the Sun and Moon are such that they appear as very nearly the same angular size from the Earth, but their apparent sizes depend on their distances from the Earth. The Earth revolves around the Sun in an elliptical orbit, so that the distance of the Sun changes slightly during a year, with a correspondingly small change in the apparent size or angular diameter of the solar disc. Also the apparent size of the Moon's disc changes somewhat during the month because the Moon's orbit is also elliptical.
An annular eclipse occurs when the Sun is nearest to the Earth and the Moon is at its greatest distance, the apparent disc of the Moon is smaller than that of the Sun. When this type of eclipse of the Sun occurs, the Moon's disc passing over the Sun's disc cannot cover it completely but will leave the rim of the Sun visible all around it.
The frequency of solar and lunar eclipses
A solar eclipse, especially a total one, can be seen from only a limited part of the Earth, while a lunar eclipse can be seen wherever the Moon is above the horizon.
In most calendar years there are two lunar eclipses (there can be up to a maximum of three) or none may occur.
Solar eclipses occur two to five times a year, five being exceptional. The average number of total solar eclipses in a century is 66 for the Earth as a whole.
From any point on Earth, on the average you will experience no more than one total solar eclipse in three to four centuries.
The situation is quite different for lunar eclipses. An observer at the same location can see an average of one lunar eclipse per year. A total lunar eclipse can last, as long as an hour and three-quarters, but for a solar total eclipse maximum duration of totality is only 7 1/2 minutes. This difference results from the fact that the Moon is much smaller in cross section than the extension of the Earth's shadow but can be only a little greater in apparent size than the Sun.
What will you see?
The first sign of the eclipse on Earth will be when the Moon's disc first touches the Sun, this is known as First Contact. The time of First Contact depends on where you are, relative to a particular eclipse. At First Contact, the Moon begins to take a "bite" out of the Sun, as the day goes on, this "bite" gets larger and larger.
As the moment of totality approaches, and the Sun is nearly covered, shadow bands might be seen; narrow bands of shadow and light racing across the ground. These are multiple images, caused by irregular refraction in the Earth's atmosphere, of the remaining "slice" of the Sun. If you are standing near or under trees, you may see multiple images of the crescent Sun being projected on the ground by the "pinhole camera" effect of the leaves.
As the Moon moves to cover the Sun, events proceed very quickly. The Moon's shadow may be seen rushing in very quickly from the west. The remaining crescent of the Sun gradually shrinks to a sliver, and then breaks up into distinct points of light, known as Baily's Beads. These are caused by the Sun shining through valleys around the visible face of the moon. Because the Moon's surface isn't equal all the way round, the display of Baily's Beads isn't consistent, but depends on the angle from which the Moon approaches the Sun.
The Moon's irregularity can also have an effect on the duration and width of the total eclipse. When only one point of light is left, a beautiful diamond ring effect may be observed, with the last brilliant point of light transfixed on the Moon's outline. Then this last glimmer vanishes, as the leading side or limb of the Moon touches the farther limb of the Sun, at a moment known as Second Contact. This is the first instant of the total eclipse.
During totality, the sky goes dark; not quite as dark as night, and with strange shadow effects caused by scattered light from the edge of the eclipse; the horizon still appears quite light, and the whole landscape takes on a strange appearance. Birds go home to roost, bees stop flying, and some flowers may begin to close as if for the night; nature seems to hold its breath. In the sky above hangs the black disc of the Moon, surrounded by a faint halo, like a negative Sun. The Sun's corona, far too faint to be seen at any time other than a total eclipse, streams out from the Moon in all directions; some streamers reach several times the size of the Sun before fading away.
For a few seconds after the beginning of totality, and again just before the end, the Sun's lower atmosphere, the chromosphere, may be seen, as a reddish glow around the edge of the Moon. Some solar prominences may also be seen, as spectacular arcs of glowing red gas around the Sun. Around the Sun, with the sky nearly dark, some of the brighter stars, and particularly planets, may be seen.
After totality the end of the total eclipse is the reverse of the beginning. Totality ends at Third Contact, the moment at which the Moon begins to uncover the Sun once again, a diamond ring, Baily's Beads, and shadow bands may be seen.
After third contact, the Sun is progressively uncovered by the Moon, and normal daylight returns. The shadow on the Sun shrinks, until it vanishes altogether at the moment of Fourth Contact.
Safe viewing of an eclipse
Warning - Observing the Sun is Dangerous. Do not look directly at the Sun, unless you are using either a solar filter or "eclipse sunglasses" specially designed to protect your eyesight.
If you do not have these filters, then project an image of the sun, through a small hole onto a flat surface, and watch the image. If you're not sure what to do, then observe with someone who does know how to safely observe an eclipse.
When the moon partially covers the sun, it is tempting to look directly at the sun. However, the sun is just as dangerous then as it is when the moon is not partially covering it. If you look directly at the sun during the partial eclipse, serious permanent damage can occur to your retina (part of the eye). It is only safe to look at a solar eclipse when the moon completely covers the disk of the sun and the corona is visible. There are safe ways of observing an eclipse, such as using a pinhole camera or projecting a telescope image onto a sheet of paper.
The totally eclipsed sun may be safely viewed without protection; but watch out for the sudden return of sunlight!
About the Eclipse
The eclipse path began on the 4th of December 2002 over the Atlantic Ocean, west of Africa. It crossed Angola, Mozambique and the Indian Ocean, before making its Australian landfall at Ceduna (South Australia) late in the afternoon (local time). The path of totality then headed northeast across the South Australian outback before leaving the Earth's surface east of Lake Blanche.
The only towns in the path of totality were Ceduna and Lyndhurst. Totality passed south of the towns of Andamooka, Glendambo and Roxby Downs, and north of Leigh Creek and Woomera.
Totality occured less than an hour before sunset for all these locations. This provided some interesting opportunities to photograph the eclipsed Sun with the local landscape.
The duration was 32 seconds at Ceduna (on the coast) and dropped to 26 seconds at Lyndhurst (northern Flinders Ranges). On the roads midway between Woomera and Glendambo, and midway between Woomera and Roxby Downs, totality was 30 and 28 seconds, respectively.
After crossing the coast, virtually all of the eclipse path was in remote and sparsely inhabited desert country.
Path of Totality
The Moon's shadow during a total solar eclipse has two parts: the umbra, where a total eclipse is seen, and the penumbra, where only a partial eclipse is seen. The umbra is quite small, perhaps as narrow as a few kilometres or as wide as a couple of hundred; whereas the penumbra is much wider, a couple of thousand kilometres or so.
This means, of course, that when a total eclipse occurs, it is only visible from a small part of the Earth, whereas the accompanying partial eclipse is seen over a much larger area. However, the shadow isn't simply a "spot" on the Earth; due to the movement of the Earth and Moon, and rotation of the Earth, the shadow actually races across the Earth's surface at around three thousand kilometres per hour, causing the Moon's shadow to "write" a long track across the Earth. This track is shown in the light blue part of the first map below.
