Observing the Inner Solar System: Mercury and Venus

February 2020  :  Dave Huestis

When guests visit the local observatories, staff astronomers always look to impress them with great views of the Moon, Jupiter, Saturn and Mars when any of these worlds are observable. The wealth of detail visible through each facility’s telescopes can awaken the sense of awe within children and adults alike. What child hasn’t marveled at the Moon’s vast craters? Who hasn’t watched the parade of Jupiter’s Galilean moons orbiting this gigantic planet and not thought about Galileo’s first view of this phenomenal sight? We sky interpreters love to hear the oohs and aahs as folks get a glimpse of Saturn’s magnificent rings for the first time. And when dust storms on Mars don’t spoil the view of this desert-like world, who can’t help but wonder if life may exist beneath its surface? Any night amateur astronomers can introduce casual stargazers to these magnificent worlds is a wonderful experience.

However, while the afore-mentioned objects get most of the glory, there are two inferior planets of our solar system that are often neglected. No, they do not have any neuroses. Inferior is an astronomical term meaning these planets orbit between the Sun and the Earth. I’m referring to Mercury and Venus. Consequently, they do not stray far from the Sun in the sky from our Earthly perspective. Examine this brief video which explains what we observe:

Whenever Mercury and Venus appear above either the eastern or western horizon these events are called elongations. Mercury can appear no more than a maximum of 28 degrees away from the Sun, while Venus can appear no more than a maximum of 48 degrees away from the Sun. Elevation above one’s horizon varies from one elongation to another.

Throughout the year we have several opportunities to observe these worlds. Unfortunately, we cannot view the surfaces of either of these planets with a telescope, but telescopically we can observe each planetary disk as it goes through phases similar to that of our Moon. Because the position angle between the Earth, Sun and Mercury/Venus is constantly changing due to our orbital positions relative to one another, we see these two planetary disks change phases. Please review the graphic at the following website: http://www.ifa.hawaii.edu/~barnes/ast110_06/rots/0520a.jpg. A picture is most definitely worth a thousand words.

Perhaps you’ve noticed a bright heavenly beacon high in the southwest sky after sunset since the beginning of the year. That’s Venus. On February 1 the goddess of love will be about 30 degrees above the horizon. On that same evening much, dimmer Mercury will be less than 10 degrees above the west-southwest horizon. You’ll require an unobstructed view to locate it. Through a telescope Venus’ disk will be 73% illuminated, resembling a waxing gibbous Moon phase.

Mercury’s disk will be about 83% illuminated and will also resemble a waxing gibbous moon phase. Mercury will continue to rise higher into the sky each evening, being at its highest elevation above the horizon on the 10^th. This date would be the optimum time to view the closest planet to the Sun. Its phase will then resemble that of a first quarter Moon. After this date Mercury will quickly sink back towards the western horizon and the Sun. Observing opportunities for Mercury are fairly short and are counted in weeks. Around March 1 Mercury will be seen in the morning sky before sunrise. On March 24 Mercury will be at its highest elevation above the eastern horizon.

After February 1 Venus will continue to rise higher and higher into the evening sky and away from the Sun and horizon. Venus’s larger orbit results in the planet appearing much farther from the Sun in our sky than Mercury does. Therefore, observing opportunities for Venus are counted in months. On March 24 Venus will be at its greatest elongation from the Sun, and therefore at its highest point (about 40 degrees) above the horizon after sunset. The phase will now look like that of a first quarter Moon. Four days later on the 28^th a waxing crescent Moon will be located about six degrees to the left of Venus. This sky scene will be a beautiful image to capture with a camera.         

It is interesting to note that Venus has been approaching the Earth since superior conjunction (passing behind the Sun from our viewpoint) on August 14, 2019. As Venus draws closer to our planet the size of its planetary disk gets larger. See this website for a graphic that illustrates this progression:   https://en.es-static.us/upl/2019/10/venus-2019-2020-ottewell-north-lg.jpg. By March 24, despite the waning phase, Venus’ brightness will remain fairly constant because its larger apparent size compensates for the decreasing illumination.

    In addition, if you know where to scan, you can even observe Venus in broad daylight, being careful not to stray too close to the Sun for eye safety. Use a building to block the Sun from direct view before beginning your sweep of the sky. However, it’s best to observe Venus in early twilight before the sky darkens. Venus is so bright that too much contrast is a problem when observed in a dark sky. A small refracting telescope or even a scope used for bird watching will show Venus’ changing phase. Check it out every couple of weeks or so.

After elongation Venus will begin to sink towards the horizon. It will still be coming towards us, all the time the phase will be decreasing to a smaller and smaller crescent. We’ll lose sight of Venus by the end of May.   

In conclusion, please remember, weather permitting, the local observatories remain open during the winter months to share beautiful views of the heavens. Snow, ice or below freezing temperatures can force closures, so please check the respective websites for any cancellation notices and observing schedules before venturing out for a visit. Seagrave Memorial Observatory in North Scituate is open every clear Saturday night. Ladd Observatory in Providence is open every clear Tuesday night. The Margaret M. Jacoby Observatory at the CCRI Knight Campus in Warwick is open every clear Wednesday night. Frosty Drew Observatory in Charlestown is open every clear Friday night.

I hope that Venus, the Roman goddess of love, will smile upon you and yours on Valentine’s Day. Just be vigilant against any errant arrows from her son Cupid!

Clear skies to all.