When is the best time to see planets in late August, with a telescope?
John S asked:
When is the best time to see planets in late August (20-30th) whilst using a telescope. I live in Southern Scotland, so thanks in advance.
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When is the best time to see planets in late August (20-30th) whilst using a telescope. I live in Southern Scotland, so thanks in advance.
August 19th, 2009 at 3:29 am
Venus, Mars and Jupiter are clearly visible right now.
This site will help you…
And this site has some nice pics…
August 21st, 2009 at 12:49 pm
The best time to see Mercury or Venus is when they are at greatest elongation from the sun. The best time to see the other planets is when they are at geocentric opposition with the sun. To predict greatest elongations or oppositions of planets, you’ll have to study celestial mechanics. The math only seems difficult from a distance. Once you get into it, it’s pretty easy.
What you’d do is use the orbital elements of the Earth, and those of the other planet, to get their heliocentric position vectors at a selected time, t. Once you have those vectors, you find their lengths and the length of their difference.
a = semimajor axis of a planet’s orbit
e = eccentricity of a planet’s orbit
i = the inclination of a planet’s orbit to the ecliptic
L = the longitude of the ascending node of a planet’s orbit
w = the argument of perihelion of a planet’s orbit
T = the time of perihelion passage of a planet in its orbit
You can find the orbital elements for every planet in the solar system at
Find the period, P, of the orbit in days.
P = (365.256898326 days) a^1.5
Find the mean anomaly, m, of the orbit at time t.
m = 2 pi (t - T) / P
Adjust m to the interval [0, 2 pi).
Find the eccentric anomaly, u. (Danby's method is shown.)
U1 = m
REPEAT...
. U0 = U1
. F0 = U0 - e sin U0 - m
. F1 = 1 - e cos U0
. F2 = e sin U0
. F3 = e cos U0
. D1 = -F0 / F1
. D2 = -F0 / [ F1 + D1 F2 / 2 ]
. D3 = -F0 / [ F1 + D1 F2 / 2 + (D2)^2 F3 / 6 ]
. U1 = U0 + D3
UNTIL |U1-U0| < 1E-15
u = U1
Find the canonical (triple prime) heliocentric position vector.
x”’ = a (cos u - e)
y”’ = a sin u sqrt (1 - e^2)
z”’ = 0
Rotate the triple-prime position vector by the argument of the perihelion, w.
x” = x”’ cos w - y”’ sin w
y” = x”’ sin w + y”’ cos w
z” = z”’ = 0
Rotate the double-prime position vector by the inclination, i.
x’ = x”
y’ = y” cos i
z’ = y” sin i
Rotate the single-prime position vector by the longitude of the ascending node, L.
x = x’ cos L - y’ sin L
y = x’ sin L + y’ cos L
z = z’
The unprimed position vector is the position in heliocentric ecliptic coordinates.
r = sqrt ( x^2 + y^2 + z^2 )
You’d do the above for both Earth and the other planet.
r1 = the distance from the sun to Earth at time t.
r2 = the distance from the sun to the other planet at time t.
d = the distance from Earth to the other planet at time t.
The angle, subtended at Earth, between the sun and the other planet, is
Q = ArcCos { (r1^2 + d^2 - r2^2) / (2 d r1) }
For an inferior planet (Mercury or Venus), greatest elongation occurs when Q reaches a maximum, which is always less than 90 degrees.
For a superior planet, opposition with the sun also occurs when the principal value of Q reaches a maximum, which should be almost, but usually is not quite, 180 degrees.
See how easy that is? If you print out my answer, or code it into a short little computer program, you will never again have to wonder when is the best time to observe the planets.
Just now (24 August 2008), it is a terrible time for looking at Saturn, since Q for Saturn is only 8.15 degrees. The glare of the sun prevents you from seeing the planet (at apparent magnitude +1.1).
Venus is an evening star with Q=20.67 degrees (mag=-3.6). Mercury is also an evening star with Q=21.65 degrees (mag=-1.1).
Mars has Q=31.06 degrees, but it’s dim (mag=+1.7) for its angular closeness to the sun, so you’d have trouble seeing it.
Jupiter is quite well positioned for observation at Q=129.68 degrees with mag=-2.3. Its disk is 95.2% sunlit.
August 24th, 2009 at 6:01 pm
Jupiter is up much of the night, but will be highest in the sky early on, say 9:00 pm. Mars and Saturn are too close to the sun to be of much use. Mercury and Venus are low in the west just after sunset - not much to see there. Later in the evening (~midnight) is a good time to pick up Uranus and Neptune.
August 25th, 2009 at 10:50 pm
dusk to dawn, depending on the planet you want to see.
mercury, venus and mars at dusk. in theory, if not in practice…
saturn isn’t really visible at the moment.
jupiter in the evening.
uranus and neptune later on.
check the charts in the magazines, or use an astronomy program.
August 27th, 2009 at 2:46 am
Right now is not a good time for observing the planets. Most of the planets (Mercury, Venus, Mars, Saturn) are low in the west just after sunset, and set soon after the Sun. Jupiter is in the south just after sunset, but very low in the sky from southern Scotland. Uranus and Neptune are visible in the late evening, but require an accurate chart to locate because of their dimness.