Tuesday, February 12, 2013

gauging visibility

Curious about something... Curious about my success rate in seeing Mercury. And some of the particulars of these events. Let's look at some of the data.

date time locale seen mag sep. alt.
Aug 12, 00 0600 rural no -1.3 10 5
May 14, 02 sunset urban ? 1.7 16 11
Jun 25, 05 sunset rural no 0.0 22 14
Nov 19, 05 sunset rural no 1.6 11 4
Jun 24, 06 sunset rural no 1.0 24 15
Aug 05, 06 0415 rural no 0.5 19 16
Feb 06, 07 1800 urban yes -0.4 18 11
Feb 10, 07 1811 urban yes 0.1 18 10
Feb 12, 07 1800 urban yes 0.5 17 12
May 18, 07 2121 urban no -0.5 18 8
May 23, 07 2000 urban no -0.1 21 25
May 27, 07 2132 urban no 0.3 23 10
May 28, 07 2121 urban yes 0.4 23 13
Jun 09, 07 2135 rural no 1.3 21 11
May 17, 08 2124 urban yes 1.0 21 10
May 24, 08 2045 urban no 1.7 17 13
Sep 02, 08 n/a urban yes 0.2 25 8
Dec 31, 08 1645 urban no -0.5 19 13
Jan 05, 09 1710 urban yes -0.2 19 11
Apr 25, 09 n/a rural yes 0.4 21 10
Jun 22, 09 0430 rural no 0.0 21 0
Oct 11, 09 0636 rural yes -0.7 16 6
Aug 08, 10 2045 rural no 0.7 27 7
Aug 11, 10 2015 rural no 0.8 27 10
Jan 10, 11 0800 urban no -0.1 23 11
Mar 16, 11 1946 urban yes -0.9 17 10
Jul 09, 11 2145 rural yes 0.3 25 6
Jul 01, 12 2116 rural yes 0.7 26 10
Feb 09, 13 n/a urban yes -0.8 16 14

Assuming naked eye or binoculars.

If I remember correctly, my first viewing of Mercury was in the mid-90s, through the telescope.

So, the statistics now. This shows I've seen Mercury as faint as magnitude 1.0, as low as 6 degrees above the horizon, and as close to the Sun as 16 degrees--but not at the same time! When it was mag 1.0, it was 10 up and 21 away. When it was 6 up, and close at 16, it was bright at mag -0.7. I've considered that as I get older, I get better at observing. So I may have simply missed it on other occasions. For example, on my first attempt, it was much brighter. But, at the same time, low and close; arguable.

I don't think these magnitude values take in account extinction...

Now, the point of this exercise to get a sense of what we might expect to see with comet C/2011 L4 Pan-STARRS in early to mid-March. I'd like to know how close an object can be to the Sun to be visible.

Mercury's a point-source, essentially. To the naked eye, star-like. A comet however should have some dimension. This is analogous to stars versus nebulae. Galaxies and diffuse nebulae and the like are "large" objects. And the magnitude value for them is for the "entire" object. If it is a very large object like the Andromeda Galaxy... The magnitude or brightness, another way to think about it, is "spread out."

Some facts on the comet.

On Mar 5, at perihelion, it will appear approximately 19 degrees from the Sun. At a predicted magnitude of 0.8. And, it will be left of the Sun. I.e. when the Sun sets, so too will the comet. So, no good for viewing then. Now, on the 16th, the comet will be above the Sun. And slightly further away, at 21 degrees. But, then, mag 1.1 is expected. Another important fact: comet C/2011 L4 Pan-STARRS originates from the Oort cloud and this will be its first trip so close to the Sun.

This magnitude numbers are from Stellarium. As of Feb 12. And the software pulled these numbers from MPC.

I dunno about you but I don't like where this is going... It is very close to the magnitude numbers for Mercury (at the time of writing). Similar angular separations and altitudes or elevations in the sky. But one's a point source and the other is not. I believe we'll be lucky to see the comet naked eye. And if the comet dims between now and then, I think we might need the telescope...

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