Venus, Jupiter and Brightness / Magnitude

Welcome back to another year of astronomy tips and news from the Urban Astronomer. I enjoy the opportunity to share astronomy with the Waldorf community and am glad to appear in the school newsletter from time to time. Throughout the year I'll talk about the changes in the night sky from season to season, share some insights about constellations and other celestial objects, provide information about special events such as eclipses and meteor showers, and of course highlight the changing positions of the planets and the beautiful arrangements of the moon, planets and stars that fill our skies throughout the year.

As we move toward Fall, the days are rapidly getting shorter and you might find yourself in the dark in the morning. Sunrise is at approximately 6:50 am which means if you are an early riser, you will have noticed brilliant Venus which is increasing in brightness and is rising due east more than two hours before the Sun. Just how bright is Venus? Astronomers use a curious system to measure the brightness of stars and planets. The system is called the "magnitude" system and it work the opposite of what you might expect, with lower numbers (and negative numbers) being brighter than higher numbers. For example, the north star (Polaris) is magnitude 2, but a brighter star directly overhead right now is Vega and its magnitude is 0. Venus is even brighter still, blazing at magnitude -4, almost the brightest it gets all year.

In the evening you can see Jupiter which dominates the southwest sky after sunset. Using the same brightness scale, Jupiter shines at magnitude -2. You can compare this to the red giant star Antares which is nearby Jupiter and although impressive at magnitude 1, seems faint compared to bright Jupiter. In the coming weeks you will notice Jupiter getting lower and lower in southwest sky, eventually fading into the sunset in a few months. Next week on Monday and Tuesday the waxing Moon will have a close encounter with Jupiter so if weather permits, enjoy the show anytime after sunset which next week is at approximately 7:15 pm.

Planets and Stars

While giving a tour of the sky, I am often asked how I know which "stars" in the sky are planets and which are real stars. The short answer is that planets are always on the go and are thus easy to spot. The word "planet" originates from the Greek word for "wandering star" and as suggested, the planets are not fixed against the background of stars in the sky but wander from place to place. Only five planets are bright enough to be easy to spot in the night sky. Because two of them orbit the Sun inside the orbit of Earth (Venus and Mercury), they move very quickly across the sky, and we only see them shortly after sunset or before sunrise. The three outer planets (Mars, Jupiter and Saturn) are visible throughout the night sky and move more slowly against the background of stars in the sky. Much like the ancient astronomers, I have come to know the basic constellations in the sky and can quickly tell when a planet is interloping in the space of a constellation. With a little investment of time, you can do this too.

The early part of this summer will be a wonderful time to get to know the planets. Venus remains a bright evening object through June and slowly fades in July. Saturn too remains visible in the evening but also fades by the end of July. And Jupiter is just beginning its reign over the summer sky as it is visible immediately after sunset in the southeastern sky. Saturn and Venus will make a remarkable pair on July 1st just on the edge of the constellation Leo. Mark your calendar for that event! If you watch over several weeks, you will notice how much change there is in the position of Venus compared to Gemini (where it is now) and Leo (where it will be in a few weeks).

If you buy yourself a good-sized star chart (10 to 12 inch), or if you buy a quality astronomy magazine such as Night Sky, Astronomy or Sky & Telescope and tear out the monthly star chart, you can get to know a few bright constellations that dominate the spring and summer sky such as Leo (where you will find Saturn and Venus), Virgo, Scorpio (where you will find Jupiter), and Sagittarius. Once you get to know these, you will be able more easily notice where the planets fit in and how they move against the background stars. By the end of summer Mars will be visible in the morning moving through Aries toward Taurus.

Make yourself a promise to learn a few constellations this summer and you'll find yourself more at home and comfortable with the night sky. And you will start to discover just where the planets are each night.

Evening Planets / Blue Moon

I spent last week at the Live Power Farm in Covelo with the 3rd grade. The sky there was clear and beautiful and we had two opportunities to meet with the kids to look at the darkening sky. At around 9:00 pm, the evening sky was filled with a parade of planets, something still visible this week as well featuring Mercury and Venus. To see this you will need a clear view of the western horizon over the next many days. The 3rd grade experienced this view and was able to see the changing position of Venus over the three evenings we were there.

Venus is moving quickly across the evening sky so if you view it on successive evenings, you will notice it moving up and to the left compared to the two twin stars in Gemini, Castor and Pollux. Further up and to the left of Venus is Saturn, currently located in the constellation Leo.

The Moon is full on May 31st, the second full moon of May for us in the western hemisphere. This unusual situation takes place infrequently and is known as a "Blue Moon." The exact time of the full moon on May 31st is 6:00 pm in California, when it is already June in Europe and Asia.Therefore, the eastern hemisphere will end up having two full moons in June. The result? We have a Blue Moon in May in North and South America, and the rest of the world gets a Blue Moon in late June. That is truly a "once in two blue moons" situation!

Moon, Scorpio and Star Parties

Now that the days are getting longer, it is getting more challenging to see the night sky in the morning. This week I was outside at 5:45 am and the sky was already brightening with daylight. So to see the skies at dawn, you will need to get up very early. Don't despair, however, because if you are a night owl and are up near midnight, you can still see the Moon drifting eastward each night across the constellation Scorpio on May 4th, 5th and 6th. Jupiter is shining brightly and will be visible just above the bright Moon. You will need a good view of the south-eastern horizon to see this at midnight, and a good southern horizon to see this in the early morning.

I would like to say "Thank You" to the Gower and Henderson families. They were the top two "Star Party" bidders at the SFWS auction last weekend and for their contribution, I'll be conducting a personal star party for them and their friends.

What is a star party, you ask? It is a gathering of people in an open space to spend quality time observing the heavens. Many astronomy clubs host star parties, events in which the members of the club assemble in a good viewing spot, set up their telescopes, and share the night sky with other members and friends. I find that amateur astronomers are very friendly people and generally enjoy sharing the wonder of the sky with others. I too enjoy sharing the sky and have conducted astronomy lectures and star parties for some of the classes at SFWS. For those who purchased my star parties, you can be assured we will have a great time learning about the motions of the stars and planets in the heavens and take a close-up look through binoculars and a telescope.

Lunar Motion

The Moon is our nearest celestial neighbor and is, from our vantage point, the fastest moving object in the sky. It circles around the Earth in 29 days and as it does presents its slowly-changing face to us at different times of night. The New Moon is the phase when the Moon is "nearest" the Sun. This takes place on Tuesday April 17th this month. Each successive day the Moon's visible area grows, at first a thin sliver and then an ever-growing disk of light.

As the Moon moves across the backdrop of stars each night, it follows a path in the sky shared by the planets and the 12 Zodiac constellations. Many of these constellations are quite bright and form beautiful patterns with the Moon as it passes by. The thin crescent Moon will be low on the horizon on Wednesday April 18th, and during the 24 hours until evening on Thursday April 19th the Moon will have moved a considerable distance across the sky, past the cluster of stars called The Pleiades, then onward just next to Venus (which itself is near the bright stars of Taurus the Bull).

Over the next week as the Moon's phase grows it will slide through Gemini (on April 22nd), next to Saturn (on April 24th) and then into Leo the Lion on the 25th with a close approach to the bright star Regulus. The Moon itself makes a wonderful object to view through binoculars or an inexpensive telescope. As the phase of the Moon changes each night, a telescope will reveal the changing pattern of shadows cast by the craters and mountains on the Moon. Take a minute to enjoy the view over the next two weeks as the Moon grows from New to Full.

The Pleiades

The winter sky is dominated by the constellation Orion which fills the southern sky during the evening hours. Orion is a guide to a few other gems in the cold winter sky such as the star Sirius, the brightest star in the skies, and the constellation Taurus which features the bright orange star Aldebaran. The three distinctive belt stars of Orion serve as the guide to these two items, Sirius by following the line of the belt stars down to the lower left of Orion, and Taurus by following the line of the belt stars up to the upper right of Orion. But even more interesting is the very distinctive star cluster just past Taurus, the Pleiades.

The Pleiades are a fascinating group of very young stars which are, relatively speaking, quite close to Earth compared to most of what we see in the night sky. The Pleiades, also known as the Seven Sisters, is an "open cluster" of stars which actually are all related to each other and are about the same age and size. The cluster formed about 100 million years ago, which in astronomical terms is recent history. For comparison, this represents about 1/40th of the time since the formation of our Sun and the Solar System. They are quite hot stars and as such are a distinctive blue color. They are all just over 400 light years away which is relatively close, a distance which is less than 0.1% of the size of the Milky Way galaxy.

During the winter you can find this cluster high in the southern sky, moving westward during the evening. As mentioned, you can use Orion's belt as a guide to find Taurus and a bit further away, the Pleiades. The brightest 9 stars have names (see diagram) which are derived from Greek mythology. The Pleiades are the seven sisters Sterope, Maia, Alcyone, Merope, Electra, Celaeno and Taygeta. Pleione and Atlas are their parents.

Close Encounters

Winter skies are beautiful for the richness of the heavens we see. The king of the winter sky is the bright distinctive constellation Orion the Hunter which rises from the south-east and dominates the southern sky during the winter months. Winter can be uncomfortably cold for long astronomy sessions, however, so rather than staying out late in the cold, I find myself enjoying the evening sky when the light changes and the stars emerge and the air has not yet cooled down. For example, on clear evenings from Ocean Beach I enjoy spotting stars and planets as darkness falls.

During the latter part of January and into February the two inner planets dominate the sunset sky. Minutes after sunset you can find Venus in the slowly fading sunset sky. It is very bright and getting brighter over the coming months. Mercury, however, is a difficult planet to find. It never gets very high up in the evening or morning sky and it never shines as bright as Venus. However, over the next week or two you will have a chance to see it twinkling in the darkening sunset sky with Venus as your guide to help find it. You will definitely need a clear western horizon for this.

Part of the joy of astronomy for me is to see these special moments when two planets pass near each other or the moon forms a beautiful pattern with stars and planets. If the skies are clear and you have a chance, enjoy this special time when Mercury and Venus share the stage.

Winter Solstice

Winter officially arrives Thursday December 21st at 4:22 pm. This is the quiet conclusion of six months of change as the Earth moves from the first point of Summer (Summer Solstice) and travels approximately 300 million miles around the Sun to the first point of Winter (Winter Solstice). On Thursday, the exact time from sunset to sunrise in San Francisco is 14 hours and 27 minutes - the longest night of the year. In the southern hemisphere of the Earth this is reversed and Thursday marks the longest day and the start of Summer. There is a lovely symmetry in the ebb and flow of the seasons. The Winter Solstice is a very important day in the ever-flowing cycles of the Sun and Earth because although the Solstice signals the completion of our move into darkness, there is hope for the future because the solstice indicates that the days are now going to be getting longer.

The skies have been changing and the cloud cover as well. If you love to look at the sky and see the stars, moon and planets, then you have been unhappy with the number of cloudy nights in December - that is, until this last week when the clear skies afforded a dazzling view of the winter sky. If the coming days allow, there will be fantastic sunset views of the Moon and Venus as they emerge from the glare of the Sun and dominate the western sky. The Moon, of course, moves quickly through its phases and as it does so, it appears higher and higher in the sky at the moment of sunset. By watching the sky from the same place on consecutive nights you can detect the motion and see the changing phase of the Moon. On the other hand, Venus is rising out of the glare of the Sun and will, over the next six months, brighten and move higher in the sky.