Notes

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Astronomy Task.
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１．） The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of the Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called theSouthern Temperate Zone. South of the Antarctic Circle, the sun is above the horizonfor 24 continuous hours at least once per year (and therefore visible at midnight) and (at least partially) below the horizon for 24 continuous hours at least once per year (and therefore not fully visible at noon); this is also true within the equivalent polar circle in theNorthern Hemisphere, the Arctic Circle.

The position of the Antarctic Circle is not fixed; as of 5 January 2018, it runs 66°33′47.0″ south of the Equator.[1] Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000-year period, due totidal forces resulting from the orbit of the Moon. Consequently, the Antarctic Circle is currently drifting southwards at a speed of about 15 m (49 ft) per year.

The Antarctic Circle is the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or partially below the horizon for twenty-four hours; as a result, at least once each year at any location within the Antarctic Circle the sun is visible at local midnight, and at least once it is partially obscured at local noon.[3]

Directly on the Antarctic Circle these events occur, in principle, exactly once per year: at the December and June solstices, respectively. However, because ofatmospheric refraction and mirages, and because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the southern summer solsticeup to about 50 minutes (′) (90 km (56 mi)) north of the Antarctic Circle; similarly, on the day of the southern winter solstice, part of the sun may be seen up to about 50′ south of the Antarctic Circle. That is true at sea level; those limits increase with elevation above sea level, although in mountainous regions there is often no direct view of the true horizon. Mirages on the Antarctic continent tend to be even more spectacular than in Arctic regions, creating, for example, a series of apparent sunsets and sunrises while in reality the sun remains under the horizon.

２．） The 2013 Eta Aquarid meteor shower was fantastic as viewed from Earth’s Southern Hemisphere. Colin Legg of Australia created this composite of his experience. He wrote, “Composite of approximately 50 images containing 26 meteors, meteor train, 17% moon, zodiacal light and Pilbara desert.”

If you’ve looked in a dark this week, you might have seen some of these meteors, although we’re hearing that they’ve been sparse so far. In 2017, the forecast calls for the greatest number of Eta Aquariid meteors before dawn on May 5 or 6. The moon is in a waxing gibbous phase and will set in the wee hours after midnight. This shower favors the Southern Hemisphere, ranking as one of their finest showers of the year. At mid-northern latitudes, these meteors don’t fall so abundantly – and twilight is long and so interferes at northerly latitudes – but mid-northern meteor-watchers who try will likely catch some Eta Aquarids, too. Follow the links below to learn more about the Eta Aquariid meteor shower.

In general, the best time to watch these fast and often bright meteors is in the early morning hours, before the onset of morning twilight.

３．） In astronomy and navigation, the celestial sphere is an abstract sphere, with an arbitrarily large radius, that is concentric toEarth. All objects in the observer's sky can be conceived as projected upon the inner surface of the celestial sphere, as if it were the underside of a dome or a hemisphericalscreen. The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or trivial.

Because astronomical objects are at such remote distances, casual observation of thesky offers no information on the actual distances. All objects seem equally far away, as if fixed to the inside of a sphere of large but unknown radius, which rotates from east to west overhead while underfoot, Earthseems to stand still. For purposes ofspherical astronomy, which is concerned only with the directions to objects, it makes no difference whether this is actually the case, or if it is the Earth which rotates while the celestial sphere stands still.

The celestial sphere can be considered to beinfinite in radius. This means any point within it, including that occupied by the observer, can be considered the center. It also means that all parallel lines, be they millimetres apart or across the Solar System from each other, will seem to intersect the sphere at a single point, analogous to the vanishing point of graphical perspective. All parallel planes will seem to intersect the sphere in a coincident great circle (a “vanishing circle”). Conversely, observers looking toward the same point on an infinite-radius celestial sphere will be looking along parallel lines, and observers looking toward the same great circle, along parallel planes. On an infinite-radius celestial sphere, all observers see the same things in the same direction.