Thomas Woodrow is an astronomy guide at Sydney Observatory and has a Masters (Astronomy) from Swinburne University. In this post, Tom discusses the planet Venus.
If you happen to be awake before 6am over the next few weeks and look to the East where the Sun is rising you will see a bright star above the horizon. This star is actually the planet Venus and is appropriately called the Morning star. At its brightest Venus is the third brightest body in sky after the Sun and Moon.
As an added bonus up until early May you will also get to see the planet Mercury. Mercury is fainter than Venus and located between Venus and the rising Sun. Mercury is the closest planet to the Sun and is usually hidden in the Sun’s glare. However, for around six weeks of every year it is far enough from the Sun, when viewed from the Earth, to be visible.
Over the coming months Venus will move east in the sky decreasing it angular distance from the Sun as we view it. By mid-July Venus will only be rising shortly before the Sun and will be lost in the Sun’s glare. Its time as a Morning star is over. When will we see Venus again?
When will we see Venus again? And why does it move the way it does?
Determining when Venus reappears is all to do with the orbits of the Venus and Earth around the Sun. Venus, after Mercury, is the second closest planet to the Sun. Viewed from the Earth it never gets too far away from the Sun in terms of angles in the sky, no more than about 45 degrees away. As it orbits around the Sun and we watch it from the Earth, Venus moves from one side of the Sun to the other alternately passing in front and then behind the Sun. When Venus is on the western side of the Sun, it rises before the Sun and is the Morning star. When Venus is on the eastern side of the Sun the planet is visible after the Sun sets and is the Evening star.
Although all the planets orbit the Sun in ellipses the orbit of Venus is the least elliptical and is very close to circular. Earth’s orbit is only slightly more elliptical. Because their orbits are nearly circular the two planets move around the Sun with nearly constant velocities which means the geometry between the two planets changes on a regular timetable. We will see that the key time signature in this relationship is a period of around 36 days.
After we lose sight of Venus in mid-July, its angular distance to the Sun continues to decrease and it is on the opposite side of the Sun to the Earth in mid-August. This arrangement of the Earth, Venus and Sun is called Superior Conjunction. Conjunction* is just a fancy way of saying ‘close together’ and means that Venus and the Sun appear close together in the sky, if you could see Venus that is!
Venus then starts increasing its angular distance from the Sun and comes out of the glare in mid-October, now on the Eastern side of the Sun and becomes the Evening star, visible after the Sun sets.
So where does the 36-day time signature appear? Well, about 216 days (which is 6 x 36 days) after Superior Conjunction, Venus reaches its greatest angular distance East of the Sun and 36 days after that reaches its maximum brightness. Another 36 days later and it stands between the Earth and Sun, lost in the glare again. 36 days later and Venus is at its maximum angular distance West of the Sun and over the next 36 days reaches its maximum brightness. Over another 216 days it moves on to the next Superior Conjunction.
If we add up all the 36-day time intervals between events it comes to 576 days which is called the synodic period of Venus. This is the period it takes for the positions between the two planets to repeat. Of course, the 36-day interval is only an approximation, although it works very well in practice, so the actual synodic period is 584 days not 576 days.
The planets of the solar system have been orbiting the Sun in a fairly regular way for a few billion years. The patterns that exist between Earth and Venus are just an example of many others that occur between the planets.
* The technical definition of conjunction is here.