Category: Lunar Events

Waxing and waning from new to full and back again

Venus and Uranus

This evening Venus and Uranus are very close, with the pair separated by a mere 19 arc-minutes, about 1/5th of a degree and well within the same medium power telescopic field. This a is chance to find the ice-giant with relatively little effort, there will be no nearby bright stars to confuse with the 5.9 magnitude Uranus. At high magnification both planets will be seen as disks, Venus 16 arc-seconds across, and Uranus a bit over 3 arc-seconds across. Venus is now far enough between the Earth and the Sun to become somewhat gibbous, being about 70% illuminated.

Venus and Uranus

Over the next few night Venus will pass very close to Uranus. The pair will be close for about five days, around 2° or less from the 7th to the 11th. It is on the 9th that the closest approach will occur with the pair separated by a mere 19 arc-minutes, about 1/5th of a degree and well within the same medium power telescopic field. This a is chance to find the ice-giant with relatively little effort, there will be no nearby bright stars to confuse with the 5.9 magnitude Uranus. At high magnification both planets will be seen as disks, Venus 16 arc-seconds across, and Uranus a bit over 3 arc-seconds across. Venus is now far enough between the Earth and the Sun to become somewhat gibbous, being about 70% illuminated.

The Moon and Pleiades

Tonight a bright gibbous Moon will be just under 5° from the Pleiades star cluster. The Moon will be 62% illuminated, bright, but the cluster is bright enough to be seen even against a bright Moon. As the Pleiades move to the west over coming months there will be a few more lunar conjunctions, with increasingly smaller crescents.

The Moon and Venus

This evening a nice crescent Moon will join Venus in the dusky sky. The pair will be reasonably close, about 8° apart. Venus is currently shining very brightly at about -4.1 magnitude, contrasting nicely with a 9% illuminated Moon. The two will still be close tomorrow, about 9° apart with the Moon 15% illuminated.

Lunar Apogee and Perigee

Does the Moon seem a little larger and brighter than usual? It may not be an illusion, sometimes the Moon really does look a little larger or smaller in the sky.

Like all orbiting objects, the Moon does not orbit in a perfect circle, but rather in an ellipse. This means that as it orbits it is a little further away or a little closer. In the case of the Moon the difference is not much, but you can see it, if you know to look.

Lunar Apogee & Perigee
The apparent size of the moon as seen at perigee versus apogee
When the Moon is furthest from the Earth, a point called apogee, it will be about 405,000km (251,000miles) from the Earth. While at perigee, the Moon will be about 360,000km (223,000miles) from the Earth, as measured from the center of the Earth to the center of the Moon. The change in distance leads to a noticeable difference in the size of the Moon as seen by an observer here on Earth. When at perigee the moon will appear about 12% larger than when seen at apogee.

Lunar Month Animation
Click to view a simulation of a lunar month showing phase, libration (tilt), and the size change as the Moon goes from apogee to perigee. Image credit Tomruen via Wikimedia Commons
The difference is most noticeable at full Moon. If the full Moon occurs near apogee or perigee, an experienced skywatcher can spot the difference. The effect should not be confused with the well known Moon illusion, where the Moon can appear larger near the horizon.

The period of time between full Moons, the synodic month, is about 29.5 days. While lunar perigee occurs every 27.5 days, an anomalistic month. Since these periods are not equal, the cycle drifts in and out of phase. About once a year the cycles coincide and full Moon and apogee or perigee will occur near the same time.

At apogee, the Moon will be appear about 29 arc-minutes in size, a little less than half a degree. At perigee the Moon will be about 33 arc-minutes across, a bit more than half a degree. The numbers may not seem like much, but it is a visible difference. The simulated images shown here will give a better idea of what the numbers represent.

This change in size and distance leads to the moonlight being a bit brighter at perigee than at apogee, about a 30% difference. So if that moonlit night seems brighter than you remember it may actually be the case.