Having explained how to locate and view Venus, Mars, Jupiter and Saturn in the beginning series, I will now progress to a lesson on Mercury, Uranus and Neptune. The first four were easy to locate--just find out which constellation they are in at any given time and look for the brightest object in that constellation. Three different techniques are needed for these three since Mercury can be seen with the naked eye, while Uranus usually requires binoculars and Neptune a telescope.

Mercury

Mercury was not excluded from the beginning series because of its magnitude. As I mentioned it can be seen with the naked eye--at magnitude 0. Its difficulty in observation stems from the fact it orbits close to the sun. As Mercury is viewed from Earth, it never gets further than about 25 degrees from the sun. Therefore it can only be viewed in twilight either in the morning just before sunrise or in the evening just after sunset. Since no stars can be seen then, star charts are useless.

Mercury orbits the Sun approximately 3 times per year. Therefore, it is visible in the morning usually three times a year and in the evening three times a year. It is easiest to locate near either greatest eastern elongation (greatest distance from the Sun) in the evening or greatest western elongation in the morning during any of these periods. You can find the dates for the greatest elongations in any astronomy magazine or calendar.

The next best time to locate Mercury in the evening will be around the next greatest eastern elongation, November 11, 1998, when the planet will be 23 degrees from the Sun. The key to being able observe Mercury is going to a location where you have a clear horizon. You need to get there before the sun actually sets. Note the path of the Sun (on the ecliptic) prior to it setting and the exact location where it sets. Mercury will not be observable with the naked eye until about 30-45 minutes after sunset. Follow the path from the point the Sun set at the horizon back along the path it had taken. Mercury should be approximately 10-15 degrees back from that point on the horizon.

If you have difficulty locating Mercury with the naked eye, try scanning the targeted area with binoculars. Once you find the bright point of light of Mercury in binoculars, follow the path down to the horizon to get a reference point. Then look above that point with the naked eye to where Mercury should be. Assuming that that spot in the sky is not cloudy and haze is minimal, the planet should appear within a short period of time if you keep staring at that spot. You can then point your telescope at it and you should be able to see it. Put in a high power eyepiece. Mercury is like Venus in that you can see no surface details. However, it also goes through phases, which allows you to see the planet during the crescent phase.

To observe Mercury near greatest western elongation in the morning, you essentially follow the same procedure. However, you may need to get up a morning or two earlier to see where on your horizon the Sun rises. Note that point and begin to look about 10-15 degrees above and to the west of that point about 30-45 minutes before sunrise on a subsequent day.

Uranus

Uranus revolves around the Sun once for every 84 times the Earth does. Therefore it stays in the same constellation for 7 years. Now it is in Capricornus.
Its magnitudes is just under 6. Technically you should be able to see it with the naked eye under dark skies, but generally there are too many stars that bright to pick it out.

Use binoculars to locate the planet. Chart 1 will help you during September and October of 1998. Start by locating Rho and Pi Capricorni. Then move left to Upsilon Capricorni and continue left not quite the same distance. You should spot Uranus easily.
It is the only object that is almost as bright as Upsilon. All other stars within the same distance are almost at least one magnitude fainter than the planet. It will move slightly during the next two months but not enough to make it difficult to locate following this pattern.

To see it with a telescope, use the lowest eyepiece you have and follow the same procedure by locating the Rho-Pi pair. Make sure you know how to move your telescope so that your are moving it left while looking in the eyepiece. You should find Uranus easy. Put in as high an eyepiece as your telescope and seeing will allow.

At a size of nearly 4 arcseconds, you should notice the difference between Uranus and a star. It should be larger than a starlike point of light. If you are good at detecting color, you should have no trouble detecting a greenish or bluish tint. Because of its distance, no other features will be observable.

Neptune

Currently Neptune is almost on the border between Sagittarius and Capricornus.

Soon it will move into Capricornus. Since the planet takes 164 years to revolve around the Sun, it stays in the same constellation on an average of 13-14 years.

You will need your telescope to locate Neptune. With a magnitude of nearly eight, you should be able to see it with binoculars. But rarely is the planet in large enough area devoid of stars around that magnitude to be identified by anything less than a telescope.
You will have to use your starhopping techniques. Like Uranus, put in a low power eyepiece and center the stars Rho and Pi Capricorni. Chart 2 has been provided to aid you. But this time move your telescope right or west until you have centered the bright star Sigma Capricorni ( mag. 5.5). Continue starhopping using the chart until you are in the area around the planet.

Stars in the chart are plotted to magnitude 9. Ticks marks show Neptune on the first of September, October and November of 1998. When you think you have located it, put in a high power eyepiece. Compare the planet to the nearby stars and the chart. The planet size is listed at 2.3 arcseconds so you may or may not be able to distinguish its size from that of a star. Try putting the suspected planet both in and out of focus and compare it with the same actions taken with a nearby star.

Neptune is also has a bluish tint which should also help confirm its identity. Like Uranus, its color is the only noticeable feature. If you are still not sure of confirmation, make a good drawing of the suspected planet and nearby stars. Do it again about a week later. The one that moved should be Neptune unless a stray asteroid moved into the field.

If you have the May 98 issue of Sky and Telescope, look on page 96 for alternative charts to help locate Uranus and Neptune. Pluto at magnitude 13 is beyond detection with most small and mid-sized amateur telescopes. Therefore I will save the last planet for the Primer for the Advanced.

NEXT ISSUE: Observing Galaxies