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Views From the Passenger Seat

How to find your way around the sky and how to plan the best Jupiter observing times 

by Dr. Leonard N. Garcia

Figuring out when and where to observe Jupiter can be difficult from this spinning, revolving planet. The Earth revolves around the Sun with 8 other planets plus millions of comets, asteroids, moons and other even smaller bits of rock and ice. Together they are called the Solar System. The Earth also spins on its axis. We can explain most of the motion we see in the sky overhead from these two types of motion.

When we step outside we are aware of the sky above us and the ground beneath us. The Earth looks flat from our perspective. The sky on the other hand looks like a gigantic dome. The line where the sky appears to meet the Earth we call our horizon. Everything that is below the horizon is blocked from view by the Earth. Now imagine a line drawn from your feet through your head and pointing straight upwards. This line would point to your zenith, the point directly overhead.

The spin of the Earth gives us our nights and days. It causes stars over the course of the night to appear to move across the sky and like the Sun rise in the east and set in the west. The imaginary line around which the Earth spins is called the axis. This line points to a particular part of the sky which is very near a star called Polaris. Polaris is also known as the Pole Star since it is close to the North Celestial Pole. The Earth's spin axis is tilted about 23.5° from a perpendicular to the plane of Earth's orbit. It is the Earth's revolution around the Sun and the tilt of the Earth's axis that gives us our seasons. It takes the Earth one year to complete one orbit of the Sun. Over a few weeks we notice the constellations are setting earlier and new constellations are rising. This is all due to Earth's orbital motion.

A sketch of the horizon and arcs used
 to describe locations in the sky Above: An illustration showing the horizon, local meridian and North Celestial Pole.

If you now draw an arc which connects your zenith with the North Celestial Pole and extends down to the north and south points on the horizon you have drawn a line called your local meridian. Whenever a star or planet crosses your local meridian we say it is transiting. Often when we talk about the location of Jupiter we may say for example, "Jupiter is at two hours before transit" meaning that two hours from now Jupiter will cross our local meridian.

The Moon

 Just as the Earth revolves around the Sun, the Moon revolves around the Earth. The motion of the Moon in its orbit can appear quite complicated because we are viewing it from a spinning Earth. The Moon moves around the Earth in just under a month (in fact our months originally were based on the orbital period of the Moon). The direction of its motion is from west to east. Because of the Earth's spin however, over the course of one night the Moon will appear to move from east to west. We can see the orbital motion of the Moon if we note the time of transit of the Moon across our local meridian each night. Each night the Moon will transit later.

How the Moon appears at sunset over
 the course of two weeks Above: An example (when facing due south) of how the Moon may look and where to find it just after sunset over about two weeks.

Of course the Moon also appears different to us depending on its location in its orbit around the Earth. This is due to the different amounts of sunlight striking the part of the Moon facing the Earth. When the Moon is low in the west soon after sunset it appears to us as a thin crescent. Over the course of about a week at the same time of night the Moon will appear higher in the sky and fuller. When the Moon transits your local meridian at sunset, it should appear half light and half dark. (the Moon is now about one quarter of the way through one orbit of the Earth and that is why it is often referred to as a First Quarter Moon). If we continue to observe the Moon night after night at just after sunset we will notice it moving into the eastern sky. Finally it will appear on the eastern horizon as a Full Moon about a week after the first quarter Moon. A full Moon will transit your local meridian at about midnight. The Moon is now halfway through its orbit. The second half of the orbit of the Moon is similar to the first. The Moon transits later in the night and less of the Moon appears lit. Finally New Moon occurs when the Moon is roughly on a line between the Earth and the Sun. This marks the beginning of a new orbit.

 The Planets

 The planets of the Solar System follow their own orbital paths around the Sun. For the planets closer to the Sun than the Earth (the inner planets Mercury and Venus) it takes less than one year to orbit the Sun. For planets farther from the Sun than the Earth (the outer planets) it takes more than a year. Jupiter, for example, takes 12 years to orbit the Sun once.

 There are certain locations in the orbits of the planets that are important for an observer on Earth. The times when Earth is between the Sun and an outer planet is called opposition. The times when the Sun is between the Earth and an outer planet is called conjunction. For the inner planets conjunction occurs when the planet is between the Sun and the Earth (inferior conjunction) or when the Sun is between the planet and the Earth (superior conjunction).

Observing Jupiter Around Opposition

 The best night to observe an outer planet is when it is at opposition. At this time the planet is above the horizon all night long. It is also at its smallest distance from the Earth. When a planet is at opposition it transits your local meridian at midnight. When listening to Jupiter with the Radio JOVE equipment it is generally (but not always) better to observe Jupiter in the weeks prior to opposition. The reason for this is that radio interference is usually less the later in the night you observe. The Sun and its effect on the Earth's ionosphere make listening to Jupiter during daylight hours very difficult. A Jupiter observing "season" should be planned for the months before and after opposition. The longer the season extends on either side of opposition the fewer hours Jupiter will be high enough in the sky to observe with the Radio JOVE equipment while the Sun is still below the horizon.

Opposition Dates of Jupiter 1995-2010
June 1, 1995
July 4, 1996
August 9, 1997
September 16, 1998
October 23, 1999
November 28, 2000
January 1, 2002
February 2, 2003
March 4, 2004
April 3, 2005
May 4, 2006
June 6, 2007
July 9, 2008
August 14, 2009
September 21, 2010
Information Courtesy: NASA RP 1349
Twelve-Year Planetary Ephemeris 1995-2006
by Fred Espenak NASA/GSFC

Left: An illustration of Jupiter in opposition with the Sun.

Right: An illustration of Jupiter in conjunction with the Sun.

An example of some planispheres
An example of a planisphere
Above: Some commercially available planispheres.

 Using a Planisphere

 Planispheres can help you plan the best times to observe Jupiter. A planisphere is a disk which shows you which stars should be visible from your location at any date and time you select. Planispheres sometimes go by the name of "Star and Planet Locator". There are many different places where you can buy planispheres. You can even build one yourself. (See the links below). A planisphere is simply a disk with bright stars and constellations drawn on it and a cover which has a window showing what stars are visible from your location. You need to be sure to get a planisphere appropriate for your latitude, otherwise the window will not show the sky accurately. Around the edge of the disk is a scale with each of the months of the year usually divided into days. The edge of the cover is marked off with the hours of the day. By turning the disk (which is attached to the cover through the center) so that the day and month line up with the hour you are going to observe the window on the cover shows the stars that are up at that time of night.

 Taking your planisphere observing with you

 The edge of the window on the cover of the planisphere corresponds to your horizon and should indicate which side is north, south, east and west. This will help orient your planisphere to the sky above. Another thing to notice is where the axis of the disk is located. Just like the sky overhead the planisphere rotates around a point very close to the star Polaris. The point in the middle of the window corresponds to your zenith and if you run a line from Polaris through your zenith and extend it all the way to the edge of the window you have drawn your local meridian. Note that the window represents your entire sky not just a small part of it. You have to imagine that all of the stars that appear in the small window of your planisphere are actually spread out across the whole sky.

Once you have found the North Pole, the zenith and the north, south, east and west points on the edge of the window you are ready to take the planisphere out observing with you. The time you use when setting your planisphere may not correspond exactly to the time on your watch. The biggest difference would be due to whether you are following daylight savings time (from early April to late October). If it is daylight savings time then you need to subtract an hour from the time you use to set the planisphere. For example if you are going to observe at 9:00 p.m. set your planisphere for 8:00 p.m. A smaller effect is the fact that the time on your planisphere represents local time and depending on where you are in your time zone you may be off (too early or too late) by up to half an hour. Turn the star disk so that the time you will be observing is aligned with your observation date. The window should now be showing the sky as it appears at that time and day. Hold the planisphere in front of you so that the edge of the window labeled with the direction you are facing is pointing down. Now see if you can match particular stars in the sky with stars showing through the planisphere's window. If it is too dark to see the planisphere clearly you may want to use a flashlight with a red bulb (this helps to keep your eyes dark adapted). Constellations near the edge of the disk will probably appear distorted so you may want to locate stars that are closer to the zenith.

Where is Jupiter Now?

 A planisphere will help you identify what stars are currently up but what about the planets? Since planets move through the constellations you probably wont find them printed on a planisphere. You may find where all of the planets are currently located by picking up a copy of a magazine like Astronomy or Sky & Telescope. Both have large maps of the sky with the current location of the planets identified. There are also web sites that can help you locate the planets. Once you find out which constellation Jupiter is in match a bright star or pattern of stars near Jupiter to the same star or pattern on your planisphere. You can use this to help you determine when Jupiter will be visible from your location. When you look for Jupiter it helps to know that it is one of the brightest objects in the night sky, only the Moon and Venus are brighter.

Try this
Refer to the table of opposition dates of Jupiter. Did you notice a pattern? What is it and why?

Hint: It takes Jupiter 12 times as long to go once in its orbit around the Sun as it takes the Earth.

Draw two concentric circles and let the inner smaller circle represent the orbit of the Earth. The outer larger circle represents the orbit of Jupiter. The Sun can be a dot in the center. Place a coin or a pebble representing the Earth on the circle you drew for the Earth's orbit. Now place another coin or pebble on Jupiter's orbit where Jupiter would have to be when it is in opposition with the Earth.

Where would the Earth be in its orbit after 1 year? Where would Jupiter be? How much more does the Earth have to travel in its orbit before Jupiter is again in opposition with the Earth?

Jupiter will be in conjunction on May 8, 2000. Try to make the same table for dates of Jupiter's conjunction from 1995 to 2010.

To Learn More
There are several sites that tell you how to build your own planisphere including ones at the University of Michigan, Otterbein and York.

Sky & Telescope has some useful information on using a planisphere and has several you can purchase.

Your Sky shows you the sky for any date, time and location you specify and includes the planets.Solar System Live gives you a view of the position of the planets in their orbits.Skymaps.com allows you to download a monthly guide to the skies.

NASA Reference Publication 1349 Twelve Year Planetary Ephemeris: 1995-2006 lists important dates for planetary observers

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