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What does 43 and 53 grade cement mean?

Grade denotes the minimum strength obtainable in a cement sample.  This is determined after making a mould of it, tom, under specified conditions on the 28th day of its moulding. The strength is expressed in megapascals (MPa).

The moulds in the shape of cubes are made by mixing cement with equal proportions of three grades of sand – coarse, medium and fine. This is cast in cubes of side about 7.07 cm so that a surface area of 50 square centimeters is obtained. Cubes cast thus are removed after 24 hours and cured in water for 28 days. Then they are subject to compression tests.

 The maximum stress in Newton/square mm which they can withstand is measured and given as their compressive strength.

If this lies between 33 and 43 N/square mm, it is termed as 33 grade cement. Similarly, if it lies between 43 and 53 N/square mm, it is 43 grade cement and if it is more than 53 N/square mm, it is 53 grade cement.  To avoid any discrepancies, always only batches of three cubes are subject to compression tests and their average is taken. Generally 53 grade cement is used for making high strength concretes. 

Could you ever see an eclipse of the Moon in daylight?

Yes, though only for a few minutes, and only if the eclipse is occurring just as the Moon is rising and the Sun is setting, or the Moon is setting and the Sun rising. A total eclipse occurs when the Earth is between the Moon and Sun, which are directly opposite to each other in the sky.

 Seeing both the Sun and eclipsed Moon at the same time would appear to be a geometrical impossibility, but because the atmosphere has a lensing effect, it raises the images of both the rising Moon and setting Sun above the horizon for a few minutes.

For the same reason, day and night are not exactly equal at the time of the fall equinox, because the day is artificially lengthened a few minutes at each end by the refractive effect of the atmosphere, for a total of seven minutes. It is not until three or four days later that the day equals the night.

It is easier to see the scallop of a partial eclipse above the horizon at sunrise or sunset than it is to see a total eclipse, because the Sun is so much brighter than the Moon.

       

With what materials are Saturn’s rings made of?

The rings around Saturn were first identified by astronomer Galileo Galilei in 1610. It was, however, Dutch Physicist Christian Huygens, who in 1659, recognized them as a broad, flat, thin ring, separated from the body of the plant.

In 1675, the Italian Astronomer G.D. Cassini identified two rings around it. Until 1969, it was believed that there were just three rings around Saturn “A”, “B”, and “C” and 151; “A” being the outermost and “C”, close to the planet. In 1969, the fourth ring was discovered by Pierre Guoria and soon, another one was also identified.

Pioneer satellite Data (1971) had indicated that was one more ring (“F”). French Astronomer, Edourd Albert Roche in 1849 postulated that the rings were the remnants of satellite that strayed too close to Saturn and due to which reason, disintegrated. His theory was that, if a satellite approaches, it’s primary, closer than a certain distance (known as “Roche Limit’ & 151; 2.44 times the radius of the planet), the satellite would break up and the broken pieces would gradually get distributed around the planet in a circular path.

The distances of the rings of Saturn are within the “Roche limit”. This would suggest that the rings are the remnants of a disintegrated satellite only.

The present thinking is that the rings are made up of countless small objects (varying in size from very small grains to small chunks of rocky material, covered by ice) and that each revolves around Saturn in its own orbit like a satellite.

That the rings contain particulate matter has been confirmed by  the fact that Rings “A” and “C” exhibit certain transparency due to which, the body of Saturn could he seen through them, as also the light from the stars.

 Furthermore, the satellites of Saturn are not completely eclipsed too, when they pass into the shadow of the rings. Very little information is available as to the precise composition of the matter in the rings. As per the observations of C.P. Kupier (1952), the Infra Red Spectrum is similar to the reflection spectrum of hoar-frost.

It is quite likely that these particles must have been of much bigger size earlier (even some metres in diameter), but these might have been a continuous reduction in their sizes due to their abrasion with objects like the meteoroids. Scientists are of the view that the continuous erosion may ultimately (at a far distant future) result in the rings slowly vanishing forever!

Why does the Moon look silvery at night?

 

 

 

 

 

 

 

 

 

 

The overall appearance of the Moon is bright ash grey caused by the dark and bright barren rocky land, where there is no atmosphere. When viewed in a naked eye or through a telescope, there are vast basins called seas, which were filled with molten lava millions of years ago. These are the low land plains appearing dark or dusky for the naked eyes. During the final ending phase of volcanism on the Moon, numerous crators had liberated enormous quantity of glowing gases and mineral vapours through their vents, which had blown in all directions over the surface, depositing the mineral condensates in the form of micron-size glassy spherules, tear drops and other powdery forms.

The Lunar surface is full of these bright rays like deposits, which scatter-reflect the sunlight quite effectively giving it a bright appearance on surface. The combined effect of rocks and soil along with the crators and minerals like calcium, aluminum and titanium therefore give a silvery appearance to the naked eye.

Why do we always see only one side of the Moon?

 

 

 

 

 

 

 

 

 

The Moon revolves around the Earth in a period of about 27 days; it also rotates once on its axis in the same time and so it always keeps the same face towards the Earth. This phenomenon is known as captured rotation.

Inspite of the fact that the Moon’s axial rotation is equal to its period of revolution round the Earth, we can actually examine more than the half of the total surface. The reason is that Moon travels round the Earth in an ellipse, not in a circle since it takes elliptical path, the rate of axial spin remains constant, whereas orbital velocity changes and moves fastest when closest to us.

We can thus see a little round alternate edges of the Moon. Also, the lunar orbit is tilted with reference to ours, so that the Moon is sometimes north and sometimes south of the mean plane, enabling us see some way beyond alternate poles. These minor shifts, known as Librations, allow us to examine four-sevenths of the total surface. The remaining three-sevenths of the Moon is permanently hidden from our inquiring eyes.

The time taken for the moon to turn on its axis once and the time taken for it to revolve once around the earth are the same. Hence the moon shows us the same face every night. This is called synchronisis rotation or captured rotation. 

 

 

 

 

 

Why do the Sun and Moon appear many times larger at the horizon?

The angular diameter of the sun and moon is about half a degree each. The celestial objects are seen with two eyes (binocular vision). When we observe the horizon, the terrestrial surface with all its objects, such as tree, houses, roads or ground give us a perspective view (geometrical phenomenon), i.e., farther the objects smaller they appear, as they subtend smaller and smaller angles at the retina.  Through this long distance perspective, which some time extends to several kilometres, our vision is able to realize a long perceptual distance. At the end of this perspective we locate the celestial objects and realize a particular size.

On the other hand, when we view these objects in the sky, the most important perspective is absent and consequently the two eyes are left with their own power to discern a distance. This binocular distance limit in the absence of the perspective has been found to be about 500 feet, which vary slightly from person to person. This can be checked by the absence of parallax shifts of objects beyond the distance when we view with alternate eyes. Therefore the sun and moon are located at short distance. Naturally a half degree object located at a distance of about 500 feet must appear much smaller  than  a similar half degree object at a considerably long, perceptual distance realized by the sense of vision with the help of perspective.

 In order to illustrate this phenomenon, we place a large ball at a distance from the eye and a much smaller ball closer to the eye in order that the smaller one just covers the angular dimension of the larger ball. Now both the balls subtend the same angle at the eye, but to our binocular vision the larger ball at that distance appears decidedly larger and it is physically larger. Through a telescope or a camera, the sun and moon appear of the same sizes either at the horizon or over the high sky. This view also applies to star constellations at the horizon and at the high sky. Monocular vision by one-eyed people has no illusion of this nature, because they have no perceptual depth. 

 

Why should not the solar eclipse be viewed with naked eye?

 

 

 

 

 

 

One should not see the Sun during eclipse because the sudden change in light intensity may damage the eye.

The human eye, as a protective measure, can adjust automatically to varying light conditions – under bright illumination, as during the day, it closes partially and under dark conditions, as in the night, it opens fully. However this process of adjustment takes some time, a few minutes.

 As it is dark during a total solar eclipse, the eye is fully open to let in more light.

After totality, as the Sun emerges the increase in light intensity is so sudden that it catches the eye unawares. Being very bright it can damage the retina even in a few seconds. 

How fast is the Earth moving through space?

The Earth is speeding like a top and like a merry-go-round horse on a carousel that is itself riding on a larger carousel, and the whole amusement park is moving through Space.

The Earth rotates on its axis at about 460 metres per second at the equator. The speed of a trip around the Sun is about 30 km per second. Meanwhile, the solar system is in a more-or-less circular orbit around the centre of the galaxy at an average velocity of about 220 kilometres a second,  according to Facts on File Dictionary of Astronomy, and the galaxy is moving at about 19.4 kilometres a second, toward a point in the constellation Hercules called the solar apex Some authorities believe that there is also a general drift by the Milky Way and some neighbouring upper-clusters towards the Southern Cross at a speed of more than a million miles an hour. 

“Why do voices differ from person to person?”

Our voice is an important part of our personality, which often identifies and characterizes us. An organ called the larynx, which is especially adapted to act as a vibrator, creates the sound of our voice. The vibrating vocal cords in the larynx, stretched and positioned by specific muscles, protrude from its lateral walls towards the centre of the glottis. During normal breathing, the folds in the vocal cords remain open to allow air to pass through easily. During speech, the folds close together so that the passage of air between these will cause vibration. Do you know that the pitch of vibration in your vocal cords determines the quality and tone of your voice? This, in turn is determined by the degree of stretch of the folds, mass of the edges of the folds, and how tightly the folds are approximated to each other. Since these three aspects vary from person to person, there are differences in people’s voices.A person can, however, voluntarily change the sound of his or her voice by changing the resonating qualities of the different stretchers of resonators like mouth, nose, and even the chest cavity. This is an art that has been perfected by actors, mimics, and ventriloquists