What's Up Tonight (04/20/2015) According To Xephem?
I don't know how to decide what to view on any given night, but when I
get the amateur astronomy itch I sit down for a brief session with my
favorite planetarium program Xephem. For me it has the right combination of utility, speed, and
These images show just some of the displays you can get from Xephem to help
you plan your evening. Whether you want to know what stars are out, what
Jupiter's moons are doing, or the phase of the Moon, these displays will tell
you. You're welcome to use this web page as a quick evening reference, but you
can plan even better if you just get Xephem. If you get
xephem on you own computer, you can adjust the view for anywhere in the world,
and step through time to see exactly when specific events will occur.
These displays are good for the evening of 04/20/2015. I've chosen
Denver, Colorado as the viewing site, and the view at 9:00 PM Denver time.
It seemed that it was a good median point of view, as Denver is centrally
located in the U.S.
This display, called the xephem Sky View, shows the position of
the stars at any selected time. This image is small to make the web page
load quickly, but with the Xephem program you can make it full screen.
You can also select different labels, and turn on display of deep space
objects (galaxies, nebula, star clusters, etc.) from various almanacs, such as
the Messier objects.
There are many other features in the actual Xephem live display. You can
right click on an item and get information about it, such as alternate names
for it, what type of object is it, or its magnitude.
When Do The Moon and Planets Rise?
The image above and left is the xephem Data Table display. It shows
some handy information to help you plan your moon and planet observing
The table shows data for the Moon, Venus, Mars, Jupiter, and Saturn. The
data listed shows each object's visual magnitude (VMag), Angular size in
arc-seconds (Size), and 24 hour clock rise time (RisTm) and set
time (SetTm) referenced to Denver Local Time.
While the sizes of the Moon, Jupiter and Saturn don't vary
enormously, the same isn't true of Venus and Mars. Size doesn't mean
a lot with Venus, as you won't be able to see details on it anyway, though
you will be able to see the planet go through phases.
As to size, pay most attention to that of Mars. Mars can vary between
about 4 arc-seconds is size, when on the other side of the sun from Earth,
to 25 arc-seconds when in a most favorable opposition.
The image at upper right shows the xephem Night At A Glance table,
which quickly indicates what items are visible at any time during the night.
Night is indicated by the dark vertical bar, the gray regions being the
Where Are The Inner Planets Now?
This xephem Solar System view shows where the inner planets are in
their orbits with respect to one another. I reduced the scope of the diagram to
only cover the inner planets since for observing considerations, their
positions more more significant.
In this view the planets orbit in a counter-clockwise direction. A
glance at this display will help you determine if the planets are going
to be morning or evening objects (also see previous image), what the
phases of Mecury and Venus will be, and whether or not Mars is near opposition.
The Phase Of The Moon
This is a very truncated version of the xephem Moon view. With the
Xephem program you can set the size of the image, turn on labels of the
Apollo landing sites, and select from a large list of lunar features which
will then be labeled on the image.
This is the way the moon would appear to the naked eye.
The View Of Mars
Star Diagonal View
These xephem Mars views show the appearance of Mars at the chosen
time. The image to the left is a representation of Mars as viewed through a
telescope using a star diagonal, such as a typical Cassegrain or refractor
telescope. The view on the right is a representation of Mars as viewed through
a Newtonian telescope. Though not visible in this reduced size image, the
Xephem live display also shows the angular size of Mars, which varies
considerably depending upon the relative positions of Earth and Mars.
This image, created using an alternate Xephem Mars map contributed by
Bob Abraham, is a graphical representation of Mars with about a 0.5 arc-second
limited view. Keep your eyes on it, as about every two years Mars and Earth
come fairly close together, and the details become more clear.
This xephem Saturn view display is also handy. It will show the
inclination of Saturn's rings at the time chosen, and display the position of
the moons of Saturn.
You can select the magnitude limit for the moons you
wish displayed, and the field of view to consider. The display here shows
a pretty large field of view to show the moon positions.
This is a typical view through a telescope with a star diagonal. The
planet rotates from right to left.
Check out the Observing Saturn web page for some hints on observing the ringed giant.
This display shows only moons as bright as magnitude 13. The table
below shows the moons' labels and typical magnitudes. Depending upon your seeing
conditions and telescope size, you may not be able to see them all.