Obs rep and notes on Saturn Venus conjunction

Our local Utah club had a small star party at our observatory (
http://www.slas.us/spoc2.htm ).

In a single low-magnification e.p. view (50mm 2" e.p. on a 5 1/2 inch
1120mm refractor), Venus and Saturn presented a great asthetic view.
Just past sunset, the Earth's terminator passed overhead, mirroring
the terminator on Venus. As the sky darkened, Saturn, Titan and Venus
became visible in a brighter twilight sky. Using an light blue (80A)
filter at higher magnification, some detail could be seen in Venus's
terminator, but the arcsec resolution of 5 1/4 inch apeture combined
with low-altitude sunset atmospheric seeing was insufficent to resolve
any other cloud detail.

The Saturn and Venus conjunction also showed a number of basic solar
system planet relationships between inferior and superior planets that
will repeat again this evening:

1) Inner planets have a phase; outer planets do not.

The inner planet Venus showed a sharp crescent phase; Saturn
presented no distinct illuminated fraction. But at conjunction, both
planets are nearly in the same visual line from Earth. Mars, which was
not observed on this evening, rises in the early morning hours and
does show a phase. See story << http://www.space.com/news/070627_mars_storm.html
.

Because both Venus and Saturn are nearly on the same sight line from
Earth, the initial common sense impression is that they should both
show a phase.

To see a distinct illuminated phase, a planet must be inferior to the
Earth or slighty outside its orbit. Each planet's Earth-planet-Sun
geometry varies due to their relative distance from the Earth. The
ratio of the diameter of the Earth's orbit (1 A.U.) to Saturn's orbit
(9.5 A.U.) is so small that the Earth never reaches an orbital
position where Saturn has a significant illuminated fraction. The
Earth-Venus-Sun phase angle is around 108 degrees, but the Earth-
Saturn-Sun phase angle is only about 4 degrees.

If the Cassini spacecraft was pointed looking back towards Earth, its
wide angle images would show both the crescent shapes of Venus and
Earth. Although no Cassini image exists, a concept simulation can be
made using the JPL/NASA Solar System Simulator << http://space.jpl.nasa.gov/
and setting the "field of view" option to 0.2 degrees.

2) Distance and physical size relate to apparent size and
brightness.

Saturn (1.0 mag), although much more massive than Venus, had an
apparent brightness much lower than Venus (-2.8 mag). Saturn is 10
times larger than Venus but is about 30 more times distant than
Venus. ( Radius: Saturn 60.2km / 6.0km ). (Distance a.u.: (9.5 a.u.
- 1.0 a.u.) / ( 1.0 a.u. - 0.7 a.u.) = 8.5 / 0.3 = ~30 ). Saturn
orbits at 1,429M km; Venus at 108 M km; and the Earth at 149.5M km.

Because of its relative nearness, the smaller diameter Venus had an
apparent linear brightness of about 28 times brighter than Saturn or
3.8 times as bright on the magnitude scale. ( 1127.2 / 34.0 = 28 ; 1v
- (-2.8v) = 3.8v ).

Because of its relative nearness, the smaller radius Venus (6.0km) had
a relatively larger apparent diameter ( radius 6km, 32") as compared
to the more distant, but much larger, Saturn ( radius 60.2km, apparent
diameter 17" ).

3) Some planets are tilted with respect to the ecliptic; others are
not.

Saturn, like the Earth is tilted on its rotation axies relative to the
ecliptic. Saturn is tilted about 27 1/2 degrees to ecliptic as
compared to the Earth's 23 1/2 degrees. Because of its cloud cover,
Venus has no visually discernable tilt, equator or rotation axis. From
radar studies is is known to be about 177 degs, or nearly aligned with
the plane of the ecliptic.

Usually observers do not perceive Saturn's tilt relative to the
ecliptic because there is no visual reference of the ecliptic plane in
the e.p. view. The rings provide a physical reference of the tilt of
Saturn relative to the Earth.

At this conjunction, mentally drawing a line perpendicular to the
cusps of Venus's terminator provides an ecliptic reference line in the
e.p. view and enables the observer to visually appreciate the tilt of
Saturn's equator relative to the ecliptic. If Venus and Saturn cannot
be seen in the same e.p. view, a reticule e.p. can be used on Venus to
set the near ecliptic line and then the e.p. can be panned over to
Saturn.

The Earth's tilt with respect to the ecliptic can be demonstrated by
referencing the angular distance between due west and the point of the
setting Sun in your local horizon system.

4) Because inferior inner planets move around the Sun relatively
faster than the Earth and the outer planets, conjunctions between
outer and inner planets are relatively more frequent.

Inferior Venus conjunctions with the outer planet Saturn within 4
degrees will occur 11 times in the next 10 years. Inferior Venus
conjunctions with the outer planet Saturn within 1 degree will occur 5
times in the next 10 years:

7/2/2007 00h 0.77 degrees
8/13/2008 19h 0.24 degrees
10/13/2009 15h 0.56 degrees
11/27/2012 05h 0.56 degrees
1/9/2016 03h 0.09 degrees

In contrast, there are no conjunctions between outer planets Saturn
and Jupiter in the next 10 years. Mars and Saturn will have four
conjunctions less than 4 degrees in the next ten years.

On 7/11/2008 06h, Mars and Saturn will be 0.69 degrees apart. In
North America, they will be low (~20 deg alt) and about due west, just
after the Sun sets.

- Canopus56

.

Relevant Pages

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