This chart provides the detail for the posting 2016-2017, Venus Evening Star.
The chart was plotted using data from the U.S. Naval Observatory’s MICA computer program. All points on the chart are calculated for Chicago, Illinois . The sunset line is the line across the bottom of the chart. The chart shows the setting times of the objects compared to sunset, beginning June 6, 2016 (Venus’ superior conjunction) and ending March 24, 2017 (Venus’ inferior conjunction). The latest setting time difference for any object on the chart is 5 hours after sunset.
When the Lines Cross
For all the objects that lie outside our planet’s orbital path, their setting times start at the top of the chart and set earlier each night until they disappear into the sun’s glare. Mercury and Venus move from evening sky to morning sky and back again. They pass behind the sun and move into the evening sky, setting higher until they reach their maximum separations from the sun and then dive between the earth and sun and move into the morning sky. This chart has two complete evening appearances for Mercury and the start of the third as Venus moves between earth and the sun.
When the setting lines of the celestial wonders cross, this indicates they are setting at the same time. It does not indicate that they are closest in their approaches to each other. The companion article, linked above describes close passings of the objects on this chart. An excellent example of this occurs early in October 2016. The setting chart above indicates that the moon and Venus set at nearly the same time on October 2, although they are closer on the next evening.
When the setting times of the celestial objects cross, they are setting at the same time, and they are likely to be close together sometime around the date of the cross. One exception is Antares. While it is near the orbital plane of the solar system, the closest approach dates can be several days before or after the simultaneous setting time. For example, the chart indicates that Mars and Antares set at the same time on August 19, 2016, they are closest on August 24 (1.75 degrees).
Planets and Stars on the Chart
Venus is represented by the green line between its two conjunction dates. During mid-January 2017, it sets nearly 4 hours after the sun. It sets during twilight until late October 2016. Locally (for Chicago) this 9 p.m. CDT. Venus reaches its greatest separation from the sun (Greatest Elongation East — GEE on the chart) on January 12, 2017. As it approaches our planet, it reaches its maximum brightness (GB) on February 17, 2017.
Mercury is represented by the brown lines that show its setting times during the Venus apparition; that is, two full evening appearances and the start of the third. It is best to observe Mercury near the time when is it near its maximum setting time difference relative to the sun. This speedy planet usually sets during evening twilight and is never seen high in the sky when the sky is completely dark.
The setting time of the moon is represented by circles (moon dots). The evening appearance of the moon starts near sunset and then sets later each night. Dates are indicated for each lunar cycle at least twice.
The stars Pollux, Regulus, Spica, and Antares make a starry background for the visible planets. They are near the plane of the solar system and the planets appear to move past them. Antares is several degrees from the plane. When a planet’s setting line intersect’s or a moon circle appears near Antares setting time, the objects set at the same time. This does not necessarily indicate that they are in conjunction.
Twilight
Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.
Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other lighting, general outlines of ground objects may be distinguishable. During nautical twilight the illumination level is such that the horizon is still visible even on a Moonless night.
Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, light from the Sun is less than that from starlight and other natural sources. For a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible. (Source)