Category Astronomy

What is the Zodiac?

            In ancient times, when people looked up at the sky in the night, they observed many bright stars arranged in a certain pattern or group. Each group is called a constellation. It means a group of stars. Studies of movements of these constellations led to the discovery that they move in space in fixed directions. The number of constellations visible during the year at different times was found to be 12. The Sun and the Moon were always seen rising and setting in the same directions in which these twelve constellations were located. These 12 constellations were called the signs of the zodiac.

            The hypothetical part of the sky through which the Sun, the Moon and other planets seem to be passing is called the zodiac. Each of the 12 constellations situated at an angle of 30°, is named as a different sign of the zodiac. In this way the 12 signs of the zodiac have come into existence. Each sign is connected with one constellation and the shape of each constellation resembles certain earthly body or substance. The names of these signs and the position of the Sun are as follows:

  1. Aries: The Ram – 21 March to 19 April
  2. Taurus: The Bull – 20 April to 20 May
  3. Gemini: The Twins – 21 May to 21 June
  4. Cancer: The Crab – 22 June to 22 July
  5. Leo: The Lion – 23 July to 22 August
  6. Virgo: The Virgin – 23 August to 22 September
  7. Libra: The Balance – 23 September to 23 October
  8. Scorpio: The Scorpion – 24 October to 21 November
  9. Sagittarius: The Archer – 22 November to 21 December
  10. Capricorn: The Goat – 22 December to 19 January
  11. Aquarius: The Water Bearer – 20 January to 18 February
  12. Pisces: The Fish – 19 February to 20 March

     

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What is lunar eclipse?

            When we stand in the sunlight, the rays of the Sun fall on us and we can see our shadows. Exactly like this, shadows of the Earth and the Moon are also formed in the space due to the Sun rays falling on them. Since the Earth and the Moon are spherical in shape, their shadows are conical in shape. These shadows are very long. The larger the distance of a body from the Sun, the longer will be its shadow. Lunar eclipse is darkness on the Moon due to the Earth’s shadow over a portion of it.

          While making their revolutions, the Sun, the Earth, and the Moon come in a straight line and the Earth sometimes comes in between the Sun and the Moon, thus the shadow of the Earth falls on the Moon. In other words, the Sunlight does not fall on the Moon when the Earth comes in between the two. The portion over which the shadow falls becomes dark. This is called lunar eclipse. Such a situation occurs only on Purnima (full Moon) days. Therefore, the lunar eclipse takes place only on that day. If the Earth’s shadow covers the entire Moon, it is total lunar eclipse. If the shadow covers only a part of the Moon, it is partial eclipse. Generally, there are three lunar eclipses in a year, out of which one is a total lunar eclipse.

             Now the question arises: when the full Moon comes every month, why does lunar eclipse not take place every month? The reason is that the plane of the Moon’s orbit makes an angle of 5° with the plane of the Earth’s orbit. Consequently, the Moon revolves either above or below the Earth’s shadow. So, it is only twice or thrice a year that all the three – the Sun, the Moon, and the Earth – come in one straight line. Hence the lunar eclipse does not take place every month. Astronomers, using mathematical calculations, easily predict the time and duration of the lunar eclipse.  

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.