31 May 2010

Continuing the discussion on the topic of Earth's orbit

Continuing the discussion on the topic of Earth's orbit, that took place at ABAA yesterday, Jayanth has sent this post for further discussion.

Table gives the calculations of earths orbit assuming same is elliptical. If this is verified by observations (Red marked values), then it a proof that the orbit of earth is not circular but elliptical.

Observational proof of Earth's Elliptical orbit



Mean period


Earth year




Mean AU


AU @ Perigee


AU @ Apogee


Year Perigee


Year Apogee


Earth Year


Earth Year Apogee


Angular velocity of earth due to rotation / day


Angular Velocity Perigee / day in degrees


Angular Velocity Apogee / day in degrees


Angular velocity Perigee translated to time sec


Angular velocity Apogee translated to time sec


Time difference in sky revolving due to earth's elliptical rotationSec


Angular velocity if earth orbit is circular


Angular velocity translated to time sec Circular orbit


Mean circular time - Perigee time


Mean circular time - Apogee time


How to observe ?

Fix a telescope pointed to a bright star of say 3rd magnitude. Lock the movements of telescope. Use a highly accurate stop watch say +/- 0.1 sec per day which is easily available. Align cross hair position on eye piece in North - South and East-West direction. Start stop watch once star centers and is on North - South cross hair. Leave the setup till next day without disturbance (Better inside the room to take care of the weather, wind, heat..............). Next day at the same time observe the same star drifting in the eye piece. Stop the stop watch once the star is on North - South Cross hair. Note the time.

It is expected to have a timing of 86400-236.5503 = 86163.4497 sec if circular.
If elliptical, depending on the position of the earth around the sun, time will vary. For Perigee it should be 86400-242.7131 = 86157.2869 sec and for Apogee it should be 86400-230.644 = 86169.356 sec

In reality, we should go the other way by observational data over the year, work back to find the Eccentricity of the orbit......... ;-))

Happy Observation


25 May 2010

Mercury Greatest Western Elongation

Mercury will be at Greatest Western Elongation on 26th Morning. The angular separation of the planet Mercury from the Sun will be 25degrees. In the early morning hours at around 5:00am, with a clear horizon, planet Mercury can be found low in the eastern horizon just before Sun rise. Mercury is in the constellation of Aries and will stay in Aries constellation till the first week of June. From today the angular separation will start to decrease, and will be very difficult to spot the planet by middle of June. If observed through a good magnification telescope, the planet Mercury will show half phase, just as half Moon phase.

Ever wondered why its called Western Elongation when the planet is seen in the eastern horizon?

Well, the answer is very simple. It just depends in which direction the planet is from the Sun. Today, if stood facing the Sun, right hand side will be north, left hand side will be south Mercury is west of the Sun, so it is said to be in Western Elongation. If the planet is in east of Sun it is called Eastern Elongation.

17 May 2010

Omicron Leonis occultation

On 20th, Thursday 2010, Constellation Leo will be good area to observe.

Planet Mars is in the constellation of Leo and on 20th, the planet is 6 degrees to the north of the Moon.

Moon will occult the 3.5 magnitude star omicron leonis, Subra, on 20th May 2010. The Moon will be at half phase and Subra is occulted from dark side of the Moon.

Here is a list of timings of occultation for few places in India.




IST = UT + 5:30




14h 36m 16s

15h 29m 49s


14h 26m 56s

15h 15m 40s


14h 49m 56s

16h 3m 0s


14h 45m 13s

15h 55m 9s


14h 30m 43s

15h 30m 12s


15h 3m 1s

15h 43m 30s


14h 44m 8s

15h 30m 47s

New Delhi

14h 33m 45s

15h 25m 13s


14h 27m 27s

15h 6m 41s


12 May 2010

Moon occults Venus

This Sunday 16th May, 2010 the two day old Moon will occult the plant Venus which is in the gibbous phase in the constellation of Taurus.

When an celestial object passes between the line of sight of an observer and another celestial object, obstructing the view is called Occultation. In this case, Moon will pass between Venus and earth obstructing the view of the Venus so we can say Moon will occult Venus.

Occultation details:(all times are in UT, IST = UT +5.5hrs)

Venus Disappears behind Moon: 10:50
Venus Reappears: 12:02

It will be difficult to spot Venus and Moon as they will be 30 degrees east of the Sun at 10:50 UT. Telescopes with setting circles or built in GOTO is advisable for viwing and photographing the event.

Venus coordinates for 16th May:

Right Ascension: 5h 40m 57.19s
Declination: +24° 47' 53.5"

Be very careful while scanning the sky for the planet Venus. Always take care not to point the telescope towards the Sun.

Venus angular diameter is 12 arc seconds and has a magnitude of -4.

Sun Position:
The Sun will be at an altitude of 31.5 degrees from the western horizon at the start of the occultation.
Sun will be at an altitude of 14 degrees above the western horizon at the end of the occultaton.

Sun set: 18:38 IST