My Science School

 

How long do the Earth and the Moon take complete one orbit around their barycentre?

27-1/3 days. If the moon’s position is observed in the sky with reference to any star (not planet), the Moon will return to the same position in the sky (with reference to that star) after a period of 27-1/3 days.  

Why does the Moon rise later each night by an average of 50 minutes?

The period of the Moon’s revolution around the Earth is not an extract multiple of the period of rotation of the Earth around its axis.

 

 

 

 

 

What do we mean by “Sidereal Period”, “Synodic Month” and “Lunation” with reference to the Moon?

The period of 27-1/3 days for the Moon’s orbit is known as the “Sidereal Period” or “Sidereal Month”.

The period of 29-1/2 days for the Moon’s phases is known as a “Synodic Month” or “Lunation”. 

 

 

 

 

 

 

What is there a difference between the sidereal period and the synodic month (Lunation)?

The answer lies in the changes in the relative positions of the Earth, Moon and Sun during the period of one orbit of the Moon. It must be remembered that while the Moon is orbiting the Earth, both are also orbiting the Sun.

 

 

 

 

 

 

 

 

What is the significance of the “Sidereal Period” of the Moon?

In one “Sidereal Period” the Moon completes one orbit of the Earth measured relative to the background stars which, for this purpose, are considered fixed in position.

 

 

 

What are syzygy and quadrature?         

The position of the Moon at Full and New Moon and the next can be divided into four parts, each part being called a quarter.

 

 

 

 

 

 

What are the features of each quarter?

During the first quarter, the New Moon changes into a crescent and then into a semicircle. During the second quarter, the semicircle changes into the circle of the Full Moon. During the third and fourth quarters, this sequence is reversed.

 

 

 

 

 

Continue reading "The Moon"

What is a New Moon?

During the period in the Moon’s cycle when it appears totally dark, the Moon is called a New Moon.

What are phases of the Moon?

The phases of the Moon refer to the appearance of the Moon when only a part of the Moon’s circle appears to be bright. Hence, the Moon exhibits phases between New Moon Full Moon. Such Moon phases can be described as Crescent Moon Half Moon etc. For example, at the half Moon phase, half the circle of the Moon appears to be bright.

What is the change in brightness from Half Moon to Full Moon?

The Full Moon is nine times brighter than the Half Moon.

What is waxing and waning of the Moon?

Waxing refers to the cyclic apparent enlarging of the bright portion of the Moon followed by waning which refers to the cyclic apparent decreasing of the bright portion of the Moon.

What is a crescent?

Crescent refers to the periods of appearance of a small bright portion of the Moon, called waning crescent and waxing crescent in the waning and waxing phases respectively.

When is the Moon said to be gibbous?

Gibbous refers to the periods of appearance of a small dark portion of the Moon, called waxing phases gibbous and waning gibbous in the waxing and waning phases respectively.

 

What is libration in longitude?

The orbital speed of the Moon varies, while its speed of axial rotation is constant. Consequently, there is a slight swaying of the Moon which enables us to see a little beyond the eastern and western edges of the Moon. This is called libration in longitude.

What is libration in latitude?

Since the Moon’s orbit is titled compared to the plane of the Earth’s orbit, the Moon periodically shifts in a north-south direction. This enables us to see a little beyond the Moon’s poles. This is called libration in latitude.

 

What is the effect of libration?

Libration enables us to see more of the Moon’s surface. Instead of seeing only 50 percent of the Moon’s surface, libration enables us to see 59 percent of the Moon’s surface. However, the remaining 41 percent of the Moon’s surface can be observed only from images transmitted from images transmitted from spacecraft orbiting the moon.

To what extent is libration represented in lunar maps?

Lunar maps are drawn based on mean libration.

 

 

Are all portions of the Moon visible from Earth?

No. only one side of the Moon is visible from the Earth at all times. The rear or “far side” of the Moon cannot be seen from Earth.

Why is the far side of the Moon not visible from Earth?

Only one side of the Moon is visible from the Earth because the Moon rotates on its axis every 27.322 days, hence keeping the same face constantly towards Earth.

How is it possible to know about the far side of the Moon?

Spacecraft launched from Earth have gone behind the Moon and transmitted images of the far side of the Moon. We know about the far side only from these images.

When was the far side of the Moon first photographed?

In 1959, the Russian Moon probe Lunik first photographed the far side.

 

 

 

 

 

What are the names of the four quadrants of the Moon?

The four quadrants are first or northwest, second or northeast, third or southeast and fourth or southwest quadrant.

Does the Moon have an atmosphere?

The moon may once have had an atmosphere. But due the low mass and consequently low gravity of the Moon, the atmosphere has slowly leaked out into space. At present, for all practical purposes, the Moon has no atmosphere.

Why is there no atmosphere on the Moon?

Every star, planet or moon exerts a gravitational force on objects on its surface. An object has to achieve a critical velocity to permanently leave the star, planet or moon. If the velocity of the star, planet or moon. If the velocity of the object is less than the escape velocity it will fall back to the surface. The escape velocity is 60 kmps on Jupiter hence the gas molecules in its atmosphere hardly escape from it. On earth, the escape velocity is 12 kmps, so which is insufficient to hold on to the gas molecules in its leaked into space. Generally an escape velocity of at least 3.5 kmps is necessary to retain the atmosphere. Mars with an escape velocity of 5 kmps also has an appreciable atmosphere and even changing seasons.

 

 

 

 

Why is the north-south direction on the Moon inverted as compared to that on Earth?

Generally astronomical maps are produced from images obtained through an astronomical telescope. Since these images are formed inverted, they can be used directly if the north-south direction is also inverted for convenience.

How do we denote north and south on Moon maps?

On a Moon map, the north is usually at the bottom and the south at the top. This is the opposite of the north-south representation on an Earth map.

 

 

How was the Moon formed?

The Moon and the Earth are thought to have been formed at the same time and in the same manner, resulting from the coalescing of the same gas and dust orbiting around the Solar System.

 

 

 

 

 

 

 

 

What is the age of the Moon?

A study of Moon rocks (brought to Earth by astronauts) indicates that the Moon and the Earth are of the same age (about 4,600 billion years old).

 

 

 

 

 

What materials is the Moon made of?

The same elements found on the Earth are found on the Moon, though in different proportions. The Moon has no air or water.

What is the internal structure of the Moon?

The moon has a core mental and crust as in the case of the Earth (though the characteristics of these zones are different). The crust has however two distinct layers- the upper crust. Also between the lower crust and the mantle is another distinct but thin region called the asthenosphere.

What is the density of the Moon?

The density of the Moon is about 60% of the Earth’s density, since heavier elements like iron occur in lesser proportion on the Moon than on the Earth.

 

What is the barycentre of the Earth Moon System?

The barycentre of the earth Moon system is the centre of mass of the Earth Moon System.

What is the significance of the barycentre of the Earth Moon System?

Through we say the Moon goes around the Earth, it is not strictly correct. Both the Moon and the Earth are actually revolving around their common barycentre.

As such, the Moon and the Earth behave as independent planets while revolving in elliptical orbits around the Sun, but they simultaneously move relative to each other around their common barycentre. Therefore, the Earth Moon System can also be regarded as a double planet system. Both the Earth and the Moon trace wobbly paths through space.

Where is the barycentre of the Earth Moon System located?

The Earth is so large compared to the Moon that the barycentre of the system is located about 1700 km below the Earth’s surface.

 

 

 

 

 

What is Selenology?

Selenology is the study of the Moon.

 

 

 

 

 

What is the Earth Moon System?

The Erath Moon System represents the system comprising the Earth and the Moon held eighty-one times the mass of the Moon and is at a distance of 384,400 km.

Let us, for example, consider a 1 kg ball and an 81 kg ball connected by a bar. The bar can be balanced at a point nearer the 81 kg ball, so the bar remains horizontal. The pint at which the bar is balanced is called the “Centre of Mass” of the system representing the two balls and the bar.

In the case of the Earth Moon System, the two balls are replaced by the Earth and the Moon and the bar is replaced by the invisible force of gravity which holds them together. The Earth Moon System will remain in equilibrium at the centre of gravity or centre of mass of the system, which will obviously be nearer to the Earth than to the Moon.

 

What is the gravity on the Moon’s surface?

It is about one sixth of the gravity on the Earth’s surface.

What is the relative mass of Earth with respect to the Moon?

The Earth has 81 times the mass of the Moon.

 

What are circumpolar stars?

Circumpolar stars (also called non-setting stars) are permanently neither visible above the horizon rise nor set. The number and type of circumpolar stars will naturally depend on the location of the observer. At the Equator, the northern and southern celestial poles would appear on the horizon, and no stars would be circumpolar. For intermediate latitudes, some stars would be circumpolar, some would rise or set and some would never be seen at all.

 

 

How do astronomers locate and refer to individual stars in the sky?

The coordinates that determine the position of a star on the celestial sphere are “Right Ascension” (R.A) and “Declination” (D.). These are polar coordinates which specify of reference of the star and determine the celestial longitude and latitude at which the star can be located.

What is Right Ascension?

The Right Ascension reference gives the celestial longitude of a body Zero Right Ascension reference is the point where the celestial equator and the ecliptic intersect. In other words, it is the vernal equinox or the point at which the Sun moves into the northern half of the celestial sphere. Considering that one circuit of the celestial sphere is equivalent to 3600 in 24 hours, we can say that 150 of Right Ascension are equivalent to 1 hour of time. The meridians at intervals at 150 are therefore called Hour Circles for the same reason. The Right Ascension reference helps the observer to align himself in the correct direction on the horizontal plane.

What is Declination?

Once the observer has aligned himself in the correct direction on the horizontal plane through the Right Ascension reference, he can locate the star by aligning himself along the vertical plane. In other words, together with Right Ascension, Declination defines the position of a body in the sky. Declination is the extent to which the star is above the celestial equator. Declination can therefore vary from 0 to 900.

 

 

 

What is correction for planetary parallax?

The coordinates (R.A. & D.) of astronomical bodies on the Celestial Sphere are theoretically with reference to the centre of the Earth. The observations of an observer located on the Earth’s surface have necessarily to be corrected by the distance between the Earth’s centre, is called correction for planetary parallax.

 

 

 

What is the ecliptic of the celestial sphere?

The ecliptic of the celestial sphere is the counterpart of the ecliptic of the earth’s sphere. Since the Earth’s ecliptic is inclined to the Earth’s Equator by 23.50 the ecliptic of the celestial sphere is also inclined by 23.50. Generally, when we say “the Ecliptic” we mean the ecliptic of the celestial sphere otherwise specified.

The ecliptic is therefore a circumferential line or great circle on the celestial sphere, titled to the celestial equator at an angle of 23.50.

What are the North Pole of Ecliptic and the South Pole of Ecliptic?

Just as the ecliptic can be visualized as the celestial equator displaced on the celestial sphere by 23.50, we can visualize the north and south celestial poles to be correspondingly displaced by 23.50 on the celestial sphere to form respectively the North Pole of Ecliptic and the South Pole of Ecliptic.

 

What is the significance of the ecliptic, the equinoxes and the solstices?

These parameters have been closely related to the changes of seasons on Earth since ancient times.

What are the equinoxes?

The two times in each year when day and night are of equal duration are called equinoxes. The ecliptic intersects the celestial equator at two points, called the points of summer or vernal equinox or “Vasanta Sampaat” (which occurs on 21st March) and the autumnal equinox or “Sharad Sampaat” (which occurs on 22nd September respectively.

What are the solstices?

The points midway between the equinoxes are called the points of summer solstice (which occurs on 21st June) and the winter solstice (which occurs on 22nd December).

 

 

 

Continue reading "The Night Sky"

 

 

Does the Pole Star have any practical significance?

The Pole Star (Polaris or ‘Dhruva’) helps navigation locate the north direction in the absence of any landmark, even in deserts and oceans. This is also called the “North Star”.

 

 

 

 

 

 

 

 

 

What are the pointer stars or “The Pointers”?

The stars Alpha and Beta (Dubhe and Marak) in the constellation Ursa Major (also known as Great Near, Big Dipper, Plough or “Saptarishi”) are called “The Pointers” since a line drawn through these stars points towards the Pole Star.

Is there any star like Polaris at the South Celestial Pole?

No. However, a group of star known as “Crux” (“Trishanku” or “Southern Cross”) is situated almost at the South Celestial Pole and helps navigators in locating the south direction.

 

 

 

 

 

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Why don’t the Sun and Moon twinkle?

Though the Earth’s atmosphere distorts the light in all cases, the twinkling phenomenon is noticeable only when the object appears very small in the sky. Even planets and large stars may appear to twinkle only when they are near the horizon. The Sun and the Moon are obviously too large for the twinkling effect to be noticeable.

 

 

 

 

 

Does nearness of a celestial body to the horizon have any effect other than increasing the twinkling?

Yes. The colour of the object turns pink, a reason why the Sun appears red during sunrise or sunset. Mercury also sometimes appears pink when seen near the horizon.

 

 

 

 

 

 

 

 

What is meant by the apparent and true direction of the Sun?

The true direction of the Sun is always different from the apparent one due to refraction of Sun’s radiation in the Earth’s atmosphere.

 

 

 

 

 

 

 

What is the Celestial Sphere?

It is imaginary sphere with the Earth as its centre and having a very large radius. The position of bodies such as stars, planets and galaxies are specified by their coordinates on the celestial sphere. In ancient times, the astrologers considered the Earth to be surrounded by the Celestial Sphere were attached to it.

Does the celestial sphere concept have any significance in modern astronomy?

No. The Earth hides part of the celestial sphere below the horizon.

What are the celestial poles, the celestial equator and celestial meridians?

The celestial sphere is assumed to posses’ poles, equator and meridians like the Earth. The point on the celestial sphere directly above the Earth’s geographical North Pole is assumed to be the north celestial pole. Similarly, the point on the celestial sphere directly above the Earth’s geographical South Pole is assumed to be the south celestial pole.

What is the celestial equator?

The celestial equator is the imaginary line representing the circumference of the celestial sphere and situated midway between the celestial poles.

What is the significance of the celestial poles?

When the Earth rotates on its axis, the celestial sphere also appears to rotate around its axis. A star situated at a celestial pole directly above any of the Earth’s poles would appears not have any lateral movement while the Earth is rotating.

 

 

What is the horizon?

The horizon is the limit of the observer’s field of vision from any point on the Earth’s surface. It is the maximum distance to which one can see across the horizon appears as a distant circular boundary surrounding the observer. The horizon recedes as one goes higher above the surface of the Earth i.e. the field of view increases.

 

 

 

 

 

How do meteors shine?

Meteors shine when they heat up and burn due to air friction while falling through the Earth’s atmosphere.

 

 

 

 

 

 

How do comets shine?

When a comet approaches the Sun, the heat from the Sun vaporizes part of the material in the comet, causing it to glow.

 

 

 

 

 

Why do stars twinkle?

Distortion of star light by Earth’s atmosphere makes the stars apparently twinkle.

Do stars twinkle uniformly in all parts of the night sky?

No. Stars twinkle more when they are near the horizon, since their light has to pass obliquely through a thicker layer of the Earth’s atmosphere which has a greater distortion effect.

 

How does the Moon shine?

The moon does not generate light of its own but reflects the light emitted by the Sun. It’s apparent large size causes it to appear brighter compared to other objects that are brighter but farther away from the earth.

Name the brightest object in the sky after the Sun.

The Moon.

Why is the clear night sky important to astronomers?

It allows visibility of almost all celestial bodies unhindered by the Sun’s glare.

 

 

 

 

 

 

 

Continue reading "The Night Sky"