28 September 2012

Sun's Angualr size as seen from different planetary distances

When we look at the sun, we always wonder how big it would appear looking at it from different planets. I have made this Image to show the relative size of Sun as seen from different planetary distances (scroll down after the table). The angular size of Sun with respect to distance is easy to calculate. Divide diameter of Sun by its distance form earth and then multiply the result by 57.3 to get the angular size of sun. Using this we can get angular size of Sun from different planets...

Angular size = diameter/distance * 57.3

Diameter of Sun = 1391000km
Distance from Earth = 150000000km

Angular size = 1391000/150000000

= 0.0092 * 57.3

Angular size of Sun as seen from earth = 0.531degrees.

We can calculate and compare the relative angular size of Sun seen from different planetary distances.

Sun Diameter = 1391000 kilometers
Distance from Sun 
(when planet is closest to sun)
Angular diameter(in degrees)
150000000 (mean distance)

Here is the a graphic of sun's appearance at different distances...

26 September 2012

ABAA News: Discussion on Kepler and Titius-Bode Law

On Sunday 23rd September, members were discussing about Kepler laws at ABAA, when calculating periods of planets, to remember the distance in AU, members spoke about Bode's law...Here is a small Jpg I made explaining Titius-Bode Law...until Uranus, numbers match beyond that the error increases...Now you will never forget the distances of Planets from the Sun :-)

Here is the Image and couple of photos of the session

Titius-Bode Law for remembering Planets distances in AU

Ravindra Aradhya explaining  Titius-Bode Law

ABAA Members at the session

10 September 2012

Venus visits M44

These days planet Venus is in the constellation of Cancer and rising early at 3am. From September 12th to 15th September Venus will pass very close to open cluster M44, popularly called Beehive cluster. Under good sky conditions, M44 is visible to naked eyes but best view is with a pair of binoculars. Take the binoculars out from 12th to 15th of September and point it to the brightest morning star, Venus, in wide angle binoculars like 10X50 M44 should be visible right-away left of Venus., in little smaller field of view binoculars like 15X70 you may have to take Venus to the right corner of the field to make both Venus and M44 visible in the same field. 

Twilight starts creeping in from 4:45am, the time before twilight should make it an amazing sight. 

Beehive cluster or M44 as its know is an open cluster has a magnitude of 3.7. The age of the cluster is approximately 700 million years old and its distance is 577 light years. Venus on the other hand is bright -4.2 magnitude and distance is approximately 140 million kilometers.

06 September 2012

ABAA NEWS: Demonstration of Planets motion in curved space.

First Sunday of September members had fun with demonstrating and discussing the motion of planets in a curved space created by massive objects like Sun. The discussions and experiments went on to different size and different density planets and also binary objects revolving around center of gravity.

These were just few updates on Sunday activities at ABAA, you can get involved by visiting ABAA on Sundays evening and take part in all the activities.

Here are few photos of the session, members called it...Come Roll the Planets...

Ravindra Aradhya

Ravindra Aradhya

ABAA NEWS: Discussions on Gravity

Last Sunday of August members had a discussion session on Gravity, The Force of Gravity was discussed in detail, motion of planets, escape velocity, gravity of Sun, tides and related topics. Every Sunday we have some discussions on Astronomy related topics, you can join us and get involved in ABAA activities.

Here are few photos of the session


ABAA NEWS: Discussions on Magnitude system.

Every Sunday evenings there are discussions on astronomy topics at ABAA, Here is a summary of discussion that took place in the middle of August. The discussion was on Magnitude system.

Apparent magnitude

Members had discussion on Magnitude system in astronomy. Starting with Apparent magnitude, the brightness of stars as seen from earth. Hipparchus, Greek astronomer grouped the brightest stars as first magnitude, little fainter stars became second magnitude, and the faintest star eye could see as sixth magnitude. The difference in brightness from one magnitude is 2.512 times, that makes 1st magnitude star is 100time brighter than 6th magnitude star.

Sirius (brightest star)
Venus (at brightest)
Full Moon
The Sun
Faintest naked eye stars

In magnitude system, negative is brighter and positive is fainter. As negative number increases the brightness increases, for example our sun has apparent magnitude of -26.8 making it brightest and Uranus has apparent magnitude of +5.7 which is fainter. 

Absolute Magnitude:

Apperant magnitude are brightness of stars as seen from earth irrespective of distances. The brightness of stars, the magnitude they appear when placed at a constant distance is called Absolute magnitude. The magnitude that stars will appear when placed at a distance of 32.6 light years which is 10 parsec( 1 parsec = 3.26ly).

Our Sun placed at this distance will have magnitude of 4.8 and Sirius, brightest star in the night sky with apparent magnitude of -1.46 will have absolute magnitude of 1.4. Members discussed in detail on these topics like calculating apparent magnitude of Sun.

The colour of sun, the topic was good in the sense that most of us think it should be yellow or red as seen from earth. But in the course of discussion it was concluded that Sun will appear white.

The topics like Luminosity, distance modulus etc were discussed in detail.

ABAA NEWS: Celestial co-ordinate system discussion

As monsoon is covering the skies, ABAA members are spending time in discussing astronomy topics and helping newcomers in understanding basic concepts in astronomy. I am updating couple of such activities at ABAA from past few weeks.

Celestial co-ordinate system discussion:

Members discussed on the celestial sphere and co ordinate systems in astronomy, getting better understanding on different types of co ordinate systems. In Equatorial Coordinate System how the declination circle is defined, the zero declination circle is projection of earth's equator. Members discussed on Right Ascension(RA) on how first point of Aries is defined and where the actual first point lies now due to earth's precession. Here are few photos of the session

Planets for the Month of September

Morning Planets:


Mercury will reach superior conjunction on September 10th and it will towards the end of the month that Mercury will be placed well for observing.


Jupiter is well placed for observations in the constellation of Taurus. The giant planet rises early at 11:45pm and has a magnitude of -2.4.  


Morning star Venus is well placed of observations through out the month. Venus rises at 3am in the constellation of Cancer shining at a magnitude of -4.2. The planet will just 2 degrees from M44 or beehive cluster as popularly known, on September 14th making it a fantastic view in binoculars. Seen through the telescope Venus will appear just more than half phase, moving towards gibbous phase as days go by.

Evening Planets:


Mars is in the evening twilight in the constellation of Libra. The red planet is at a magnitude of 1.2 and will set at 9:20pm.


Saturn is also placed low in the Western horizon, in the constellation of Virgo. The planet sets early at 8pm.


Uranus is well placed for observetion through out the month. Uranus rises early at 7pm giving full night for observations. This planet is visible through a pair of binoculars is at a magnitude of 5.7. Uranus will reach opposition on September 29th.


Neptune is also well placed for observations through out the month in the constellation of Aquarius. Neptune has a magnitude of 7.8.

Moon Phases:

Last quarter: 08-Sep-2012
New moon: 16-Sep-2012
First quarter: 22-Sep-2012
Full moon: 30-Sep-2012


On September 22nd Sun will reach autumnal equinox, when Sun's declination becomes zero and Right Aessionsion(RA) becomes 12 h. The Sun will reach this co ordinates at 9pm IST on September 22. Although this point is called First point of Libra, the Sun will be in Virgo constellation.