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9.2 Using PlanetChart
Basic information on how to read the PlanetChart output has been given in Section 2.2. There are, however, many other details and features built into the chart.
All planets, as well as the Sun and the Moon, move along the ecliptic indicated by the yellow line in the star field at the bottom of the chart. The star field shows a 360 degrees swathe of sky for the region 36 degrees above and below the celestial equator. Only half of the stars shown in the star field are visible at any given time, however; the rest are below the horizon. In general, the east and west horizons correspond to roughly vertical lines in the star field. Those horizons move slowly to the left, with increasing right ascension, as the night goes by. For each hour of time, right ascension increases by one hour as well.
To orient the star field, first turn so that your back is to the Pole Star if you are viewing from the northern hemisphere. (If you are in the southern hemisphere, turn so that your back is to the Southern Cross.) Next hold the star field above your head with the top (bottom) pointing back over your head to the Pole Star (Southern Cross). Then, in front of you, in a great circle about 90 degrees away from the Pole Star (Southern Cross), you should see approximately half the stars shown in the star field, with the other half below the horizon. This can take some practice to do quickly, so first try finding the constellation of Orion, which is in the center of the star field with a right ascension between 5h and 6h. At some time during the night Orion is visible above the horizon, as are all the constellations in the star field. The transit table near the bottom of the chart should help you find the stars in the star field that are directly in front of you at various hours in each month of the year. Stars are said to transit when they are directly south (north) of your location. If you orient the star field correctly, then, for example, at 23:00 (or 11pm) each night during the month of January you should be facing the zodiac constellation of Gemini. Two hours later, at 01:00 (or 1am), you should be facing the zodiac constellation of Cancer, and so on. These times may need to be adjusted if you are using daylight-saving time, and if you are not in the exact center of your time zone.
To get a sense of scale, two hands are shown in the star field, separated by 90 degrees. For most people, a hand span (measuring from index finger to little finger), when held at arm's length, subtends an angle of about 15 degrees, or 1 hour of right ascension. This is illustrated to scale in the star field. The constellation of Orion covers a solid angle approximately equal to the size of your hand at arm's length.
To get a sense of how bright the planets are, note that the three stars forming the belt in the constellation of Orion are each nearly magnitude 2, and the two brighter stars above the belt are magnitudes 1 and 2. The two brighter stars below the belt are magnitudes 2 and 1. Also note that Sirius, the brightest star in the sky, is brighter than magnitude -1. Sirius is the bright star one hand span away from Orion, with a right ascension of about 7h. Most of the major planets are usually quite bright and easily found if you know where to look. For example, Mars is often much brighter than the brightest stars in Orion, Jupiter is considerably brighter at about magnitide -2, and Venus is even brighter still at magnitude -3 or -4, depending on its phase. When visible, Jupiter and Venus are the brightest objects in the sky, with the exception, of course, of the Moon, which is obviously much brighter at approximately magnitude -12. Saturn and Mercury are as bright as the brightest stars in Orion. In the key at the bottom of the chart, the mean magnitude of each major planet is displayed as a dot along an ecliptic line. You can directly compare these planet dots with the star magnitude dots used in the star field.
The planet chart has diagonal lines labeled with hours either before sunrise or after sunset. To make reading the chart easier, the words "Morning" and "Evening" are shown to indicate whether the planets are visible in the morning before sunrise or in the evening after sunset. For example, on 1994 September 1, the planet Jupiter is visible in the evening sky. By reading the diagonal scale you see that it is visible for about 4 hours after sunset before it sets in the west. If a planet is along the line of full moons, it is at opposition to the Sun and is visible the whole night, whereas if it is within 1 hour of the Sun, it is generally not visible because of the glare after sunset or before sunrise.
Other features to note are that the half moon crescents are facing in the correct direction in relation to the star field, and that lunar eclipses are indicated along the full moon line. The lines for the outer planets, Mars, Jupiter, and Saturn, are always drawn behind the Sun's yellow line, which is correct since these planets always pass behind the Sun. However, for the inner planets, Mercury and Venus, the lines may be in front of the Sun as well. The thickness of a planet line is meant to convey a sense of apparent brightness or magnitude, which is why the Venus and Jupiter lines are thick, and the Sun's line is even thicker.
The planet chart contains some necessary simplifications. First, a given planet will not always lie exactly on the line of the ecliptic in the star field because its orbital plane may be slightly inclined to that of Earth's. The maximum departures, either above or below the ecliptic, for each planet are: Mercury, 3°; Venus, 3°; Mars, 2°; Jupiter, 1°; and Saturn, 2°. These are all fairly insignificant angles in the star field. The Moon, however, may be as much as 5° above or below the ecliptic, which is a more noticeable effect. A second simplication involves the transit table, which, as mentioned earlier, assumes that you live in the center of your time zone. The potential error is only 30 minutes of time, since in theory no one lives more than 7.5 degrees of longitude from the center of a time zone. You will, of course, have to add one hour to the transit times when you are experiencing daylight-saving time. A final simplification involves the dots representing the brightness of each planet shown in the key at the bottom of the chart. During the course of a year, the brightness of Venus and Mars will vary significantly because these planets are in orbits close to Earth. The dots for Venus and Mars, therefore, simply represent the mean brightness; the true brightness depends on how close the planet is to the Earth at the time of viewing.
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