The Movement of Stars

For thousands of years ground based observers had speculated from their understanding that stars were immobile and that they were stationary relative to Earths perspective. This curiosity of mankind to ask such questions as, how far is that star? how big is it? and how bright is it?. These questions have brought great discoveries. Our nearest star Alpha Centauri also known as Rigil Kentaurus or Toliman is at a distance of 4.22 light-years. Rigil Kentaurus comes from the Arabic term meaning 'centaur's foot', this figure was visualized by Arab astronomers in the stars of this region.


With 1 Ly equal to 9.4605284 × 10¹² km, 4.22 Ly would make it a journey of 3.992342985 x 10¹³
km, making it a very long journey even with our current technology it would take over 100 years to reach our nearest star. Considering our galaxy the Milky Way is estimated to have around 400 Billion stars reaching our closest neighbor is still yet so far.


In 1718 research from English astronomer Edmond Halley proved the proper motion of star, he observed three bright stars Sirius, Arcturus and Aldebaran.
Proper motion is the angular change in position of a star across our line of sight on Earth. It is measured by astronomers and scientists by optical methods of photography several years apart and then measuring the movement of a star with respect to the stars in the distance or behind it over a period of time.



Sirius which is the brightest star in Earth's night sky with a magnitude of -1.46, is located in the constellation of Canis Major south of the celestial equator. It is perceived to be a single star but in fact is a binary star system consisting of Sirius A and Sirius B. Sirius is moving closer to our Solar System and will slowly increase in luminosity over the next 60,000 years.

[Simulation of orbit of Sirius A and Sirius B, Sirius A on right and Sirius B on left]

Arcturus is a star with a visual magnitude of -0.05 at a distance of 37 Ly. It is fourth brightest star of the northern hemisphere and is estimated to be about 25 times our Sun's diameter and 100 times as luminous.

[Simulation of Arcturus]

Aldebaran is a orange giant star about 65 Ly away. It marks the eye of Taurus the bull. It is about 150 times more luminous then the Sun. The name Aldebaran comes from the Arabic meaning 'the follower', it appears to follow the Pleiades cluster across the night sky.

[Aldebaran with the Pleiades cluster behind it]

Edmond Halley noticed that these three stars were more than 0.5 degrees away from previous positions given by the Greek astronomer Hipparchus who is famous for his discovery of the equinoxes in the second century BC. This shift of 0.5 degrees was apparent in the sky if observed over time. This is now called proper motion, which is defined as the change of position of a star on the celestial sphere. Proper motion is denoted by the Greek symbol Mu.


Despite being most famous first to calculate the orbit of a comet named after him Halley's comet. The discovery and study of proper motion is critical to understanding the celestial bodies of stars in our universe. It took 2000 years for this motion to change and become apparent to scientists. With this we can see that stars that have a large proper motion as with Barnard's star tend to be nearer to our position and stars that are further away have a much smaller proper motion.

Barnard's star is perhaps the best example of proper motion it travels 10.3 seconds of arc each year, an arc is a unit of angular measurement equal to one-sixtieth (¹⁄₆₀) of one degree

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Until next time goodbye for now.

The solar eclipse March 20th, 2015

[Picture of solar eclipse March 20th 2015 9:20am, Picture by Sidra Malik, Dundalk, Ireland ]

Millions of people around the world witnessed a spectacular phenomenon, the solar eclipse 2015. The Moon passed directly in front of the Moon to bring a solar eclipse to observers. Across United Kingdom, Ireland and eastern Europe experienced a partial eclipse. The Moon covered about 60 percent of the Sun.

[Simulation of solar eclipse viewed from space]

One of the most prominent features of the sky is the Moon. Over the thousands of years man has looked up at the night sky and has always seen he same side of the Moon. However the one thing that does change in appearance, is the sides that are illuminated so its appearance changes regularly throughout the day and seasons.

Whether it is today, tomorrow, next week or next year, the same side of the Moon and same patterns will still be observed. Many people make a false observation by thinking the Moon does not rotate however in fact the Moon does rotate on its axis. It rotates once for every revolution it makes around the earth and always keeping the same face towards the earth. This is quite an amazing phenomenon called synchronous rotation as the revolution and rotation are synchronized with perfection. The rotation is slightly elongated causing the near side always face towards the earth.

[Simulation of rotation of Moon around Earth]

The other side of the Moon which we seldom see in fact the far side or dark side of the Moon spends around the same amount of time in sunlight as the near side. This was unknown to man until recent advancement in space flight and technology.

[(simulation) Far side of moon receiving light from the Sun]

"Blessed is He Who made constellations in the skies (Space),
And placed therein a lamp (Sun)
And a Moon which has reflected light." - Al Quran

Unlike the Sun, the Moon does not have a source of light of its own, it reflects and shines by the light  received from the Sun.

There are three types of solar eclipse; total, partial and annular. A total solar eclipse occurs when the Moon completely blocks the disk of the Sun. This is one of the most amazing sights man can witness in nature.
A partial solar eclipse occurs when the Moon only partially covers the Sun.
An annular solar eclipse occurs when the Moon is slightly further away from the Earth in its orbit and appears smaller in sky, it goes over the sun but does not completely block it.

   [A total solar eclipse]

   [A partial solar eclipse]

   [An annular solar eclipse]

However you may be wondering despite all of this how is it that the Moon and the Sun seem to be the same size ?

This is due to a remarkable phenomenon by where the Sun and Moon appear to be the same size when we view from Earth. The Sun has a diameter which is about 400 times greater than the Moon but at the same time the Sun is also about 400 times farther away. The diameter of the Sun is 1,391,684 km, distance of Sun from Earth is 149,600,000 km. The diameter of the Moon is 3,476 km with a distance of 384,400 km.

A remarkable coincidence indeed!

One of the important astronomers of the years 858-929 was Al-Battani, Abu'Abudallah Muhammad ibn Jabir also know as Albateginus. An Arab astronomer who demonstrated that the Sun's distance from the Earth, its apparent angular size, varies which explained why both lunar and solar eclipses are made possible. He made the first truly accurate calculations of the solar year (365.24 days). 

[Crater on Moon named in honor of Albateginus]

He also produced a number of important trigonometric equations that are now commonly used across many disciplines of Engineering and other Sciences . Data produced by Albateginus continues to be used even today.


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To see a total solar eclipse you must be located at at a very narrow band where the Moon's shadow is cast onto the face of the Earth.
NASA's SDO (Solar Dynamics Observatory) which makes its path around earth in 24 hours is situated in a geosynchronous rotation around earth and is in unique situation where it views an eclipse where the Moon is covering the Sun and another eclipse where the Earth eclipses the Sun. 

We are not as lucky to see such eclipses simultaneously but the next eclipse to occur and be visible from Ireland will be in the year 2026 and hopefully it wont be cloudy day with such poor visibility as we had this time, so until next time goodbye!