Archive for February, 2010

Useful astronomical numbers

Posted February 19, 2010 By grigoris

Just some useful numbers and units to remember and to have an easy access to!
[last updated: 29 Dec 2011]

>> Distance

1 AU = 150 x 106 km

1 pc = 3.09 × 1013 km = 206260 AU = 3.26 ly

1 ly = 9.4 x 1012 km = 64 x 103 = 0.31 pc

>> Sun

L☉ = 3.86 x 1026 W = 3.86 x 1033 erg / s

MV,☉ = Mbol,☉ = 4.8

M☉ = 2 x 1030 kg

R☉ = 6.96 x 105 km

V☉,relative = 30 km/s

T☉ = 5780 K

RSun-GC = 8 kpc

tgalactic orbit = 250 x 106 yr

Vgalactic rotation = 200 km/s

solar wind’s energy ~ 10-7 L☉ (on average)

solar wind’s density (in the vicinity of Earth) ~ 5 particles / cm3

solar wind’s velocity (in the vicinity of Earth) ~ 400 km / s

mass loss ~ 10-14 M☉ / yr

>> Earth

MEarth = 5.974 x 1024 kg

REarth = 6371 km

ρEarth = 5.5 g/cm3

>> Milky Way / Galaxy

bulgeradius ~ 5 kpc

diskradius ~ 15 kpc

haloradius > 75 kpc (perhaps 100 kpc)

dust ± 300 pc from disk level

stars ± 1000 pc from disk level

MMW = -20.5 ( 25 mag more than MSun, 1010 times that of Sun)

>> Ionization potential

for H = 13.6 eV

for He = 24.6 eV (one electron) + 54.4 eV (both electrons)

>> Hydrogen atom

R = 1 Å

>> Extinction

Sun’s neighbourhood: ~ 1 mag/kpc

towards GC: ~ 21 mag/kpc – 1 optical photon per 1012 reaches us, while 1 out of 10 in IR

>> magnitude difference: 5 mags = 100 times in flux [m1-m2=-2.5 log10(f1/f2)]

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About full Moon and Moon illusion

Posted February 13, 2010 By grigoris

The size of a Full Moon depends on the position of the Moon in its orbit (~ 0.5 degrees). Thus the biggest Full Moon is when the Moon is closest to Earth (i.e. at perigee).

from spaceweather.com (28 Jan 2010)

“… a “perigee Moon” some 14% wider and 30% brighter than lesser full Moons of the year.

Johannes Kepler explained the phenomenon 400 years ago. The Moon’s orbit around Earth is not a circle; it is an ellipse, with one side 50,000 km closer to Earth than the other. Astronomers call the point of closest approach “perigee,” and that is where the Moon will be Friday night.

Look around sunset when the Moon is near the eastern horizon. At that time, illusion mixes with reality to produce a truly stunning view. For reasons not fully understood by psychologists, low-hanging Moons look unnaturally large when they beam through foreground objects such as buildings and trees.”

(Check also about Moon illusion on wikipedia.)

from APOD (25 Oct 2007)

“… The difference in apparent size between the largest and smallest Full Moon is quite dramatic and similar to this side by side comparison of the lunar apogee/perigee apparitions from 2006. But seen in the sky many months apart, the change is difficult to notice. “

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A great image of star sizes

Posted February 12, 2010 By grigoris

A magnificent picture comparing star sizes !!! (from Wikipedia / Star – Diameter)

Star sizes

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Moon (Lunar) phases

Posted February 12, 2010 By grigoris

Just a quick note for moon (or lunar) phases.

Moon (Lunar) Phase Diagram

From Wikipedia / Lunar Phases (moon phases).

Phase Northern Hemisphere Southern Hemisphere Visibility
New moon Not visible, traditionally Moon’s first visible crescent after sunset
Waxing crescent moon . Right 1-49% visible Left 1-49% visible afternoon and post-dusk
First quarter moon Right 50% visible Left 50% visible afternoon and early night
Waxing gibbous moon Right 51-99% visible Left 51-99% visible afternoon and most of night
Full moon Fully visible Fully visible sunset to sunrise (all night)
Waning gibbous moon Left 51-99% visible Right 51-99% visible most of night and morning
Last quarter moon Left 50% visible Right 50% visible late night and morning
Waning crescent moon Left 1-49% visible Right 1-49% visible pre-dawn and morning
Dark moon Not visible, traditionally Moon’s last visible crescent before sunrise

For the intemediate lunar phases keep in mind that:

crescent : less than half illuminated

gibbous : more than half illuminated

waxing : becoming more luminous

waning : becoming less luminous

So, by combining the terms you can figure out where the moon lies in its orbit and what the phase will be:

> after new moon, the moon becomes more luminous than before, but still less than half in total = waxing crescent

> after first quarter, the moon contimues to raise its luminosity but more than half = waxing gibbous

> after full moon, luminosity drops but still more than half is illuminated = waning gibbous

> after last quarter, luminosity drops even more (towards full moon) but now the total illuminated area is less than half = waning crescent

Moon phase calenders:

1. www.moonconnection.com – current phase moon & calendar

2. U.S. Naval Observatory – moon phase images

3. U.S. Naval Oceanography portal – phases of the moon

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40 characters limit in Starlink

Posted February 3, 2010 By grigoris

When you install Starlink you have to determine the directory of the program (set env starlink_dir /path/to/software/star, in the ~/.tcshrc file). Then, any application runs under this directory.

But, there is a limit in the number of characters used (only 40). This can cause some programs not to run at all or run very quickly (and not producing any output of course!).

So keep an eye on the length of your working directory !

[2011/05/05 NOTE: this should not be valid after Hawaiki version !]

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While loop in C-shell scripts

Posted February 3, 2010 By grigoris

When using the while loop in C-shell scripting the usual stucture is:

while (expression)
commands
…
end

BUT, you need to add an empty line or a comment after end in order for the loop to work!

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