The crescent Moon close to Venus on 10 August 2013. Photo Nick Lomb
A favourite sight in the night sky, visible even to people in light-polluted cities, is when the crescent Moon is near the bright planet Venus. These two objects are the brightest in the night sky and can be seen in the twilight sky before anything else becomes visible. The encounters between the crescent Moon and Venus occur in a regular monthly pattern; if Venus is visible in the evenings after sunset then the encounters occur in the evenings, while if Venus is visible in the mornings before sunrise then the encounters occur in the mornings.
An Excel diagram showing the setting times of Venus and the Moon from the beginning of August to the end of December 2013. Diagram Nick Lomb
One way of predicting the monthly encounters of the crescent Moon and Venus is to look at their setting times. In the above diagram Venus (blue line) sets in the relatively narrow range of approximately 8 pm to 11:20 pm with a one hour jump in early October due to the start of daylight saving. In contrast, each month the setting time of the Moon spans the full gamut of hours in the day with a steady increase of setting time by about 50 minutes each day.
We can predict when the Moon is to be near Venus by looking at their setting times. When the setting times of the two bodies are the same or similar we know that they must be close to each other. In the above diagram the setting times are close or the same whenever the Moon setting curve intersects the curve relating to Venus. It is clear from the diagram that an encounter will take place each month.
Diagram showing the relative positions of the Sun, Venus, the Earth and the Moon on 10 August 2013. The motion of the planets and the Moon are anticlockwise. The orbit of the Moon has been exaggerated for clarity. Diagram Nick Lomb
As previously discussed, Venus shows phases just like the Moon. This observation leads to the question, do Venus and the Moon show the same phases during an encounter? The answer is that their phases are different as the geometry between the Sun, the Earth and Venus or the Moon is different. This can be seen from the diagram above where Venus is situated so that just over half its disc is illuminated by the Sun as seen from Earth. Hence Venus is at a gibbous phase, although over the next few months Venus exhibits a steadily diminishing phase until it is in the same direction as the Sun on 12 January 2014. The situation with the Moon is different as it appears much closer to the Sun and so we see it at a crescent phase. It should be noted that as Venus never strays far from the Sun as seen from Earth, the Moon must necessarily be at the crescent phase during its encounters with Venus, whether in the evenings or in the mornings.
The next encounter between Venus and the crescent Moon is on 8 September 2013. Enjoy the view!