31 July 2014

August Opens With an Evening Show

Evening Show
The waning crescent Moon creates a majestic skyscape to open the month of August, shimmering against the dusk sky with the shiny colors of Spica, Mars and Saturn in the path. Each evening the trio of planets and stars will have the Moon in their midst, and the Moon will slip gradually eastward with each successive night.

I enjoy seeing the waxing Moon with its delicate shape and edge-on illumination from the Sun. It's fun to see in a telescope and it always holds the promise of the gentle glow of Earthshine. In the middle of summer, the ecliptic is low on the southern horizon and the Moon slices a gradual slope across the sky.

This view will be enhanced by binoculars, if you have them. You will see deeper into the details of the Moon and into the constellations along the southern horizon with the fringes of the Milky Way in view due south as darkness falls.

Image courtesy of Sky & Telescope.

30 July 2014

Sun and Moon and Ebb Tides

A kayaking friend of mine referred me to a question posed by a fellow kayaker regarding tides in and around the Golden Gate: Why are Ebb Tides strongest in SF Bay at night during the summer and during the day in winter? I love this kind of question, where the celestial mechanics of the Solar System impact the daily experiences of hobbyists and average people on Earth. 

First of all, why are there differences in size of high & low tides throughout the month and year? Each month the highest highs and lowest lows occur when the Moon is Full or New. That’s pretty simple. But there are two important factors regarding the Moon’s proximity to Earth on any given Full or New Moon. And there is one important factor regarding the Sun’s proximity to Earth on any given Full or New moon.

1. The Sun’s Changing Proximity

The Earth’s orbit around the Sun is elliptical, meaning that there is a close approach each year on January 4th (“perihelion”) and a corresponding far point in our orbit in July (“aphelion”). The New and Full Moon phases just before or after January 4th have higher high tides at the noontime tide, leading to a very fine ebb tide in the middle of the afternoon.

2. The Moon’s Changing Proximity

The Moon’s orbit around the Earth is elliptical, so there are times when the Moon is closer (“perigee”) and farther (“apogee”). Lately the press has made much to do about the “Super Moons” that occur when the Full Moon lines up with Perigee. On August 10, 2014 we will have a very fine Super Moon precisely at the same time as Perigee, so there will be higher high tides at midnight and lower low tides at dawn, leading to a very fine ebb tide in the middle of the night.

Another factor is the Moon’s location along its orbit from a north-and-south perspective. The Moon’s orbit around the Earth is inclined slightly and follows the “ecliptic” which also meanders above and below the plane of the equator, meaning that sometimes it is physically above the plane of Earth’s equator and sometimes below (“ascending and descending nodes” of the lunar orbit). If the Moon is above the plane of the equator, it is actually a bit closer to the land and water north of the equator, exerting a slightly greater tidal pull. The next peak of this effect is on August 9, 2014, so we can look forward to some amazing high and low tides at the next Super Moon. The ascending node and its impact are not tied directly to winter or summer, so this effect is less predictable.

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To answer the original question: there are good ebb tides each month at New and Full Moon, but the changing effects of perihelion and perigee cause the solar influence to be greater in Winter (hence the better daytime ebb tides) and the lunar influence to be greater in Summer (hence the better nighttime ebb tides).

This website offers additional insights into tides. 

Fun subject to think about! Wow!

20 July 2014

Binocular Astronomy

From an urban setting such as my home in San Francisco, I have a reasonable view of the night sky, knowing that from my own backyard I can see a good number of stars if I have clear skies and I am patient, allowing my eyes to dark adapt. In a city setting you are limited in the depth of the night sky you can experience by the ambient light in your exact setting, and the light pollution dispersed into the sky. But you can overcome these factors somewhat, by using binoculars.

Every time I am at a star party or astronomy gathering, in addition to a telescope I bring my binoculars. These are the fastest way to enhance your viewing whether you are in a dark sky or city setting. No matter what conditions you have, you will see deeper and will experience more richness in the night sky with binoculars. They are intuitive and require no special technical knowledge to use. You just point at a part of the sky and enjoy. Gary Seronik of Sky & Telescope publishes regular articles focused on binocular viewing and has an excellent resource book (I have a copy, of course) just for binocular viewing, Binocular Highlights. I highly recommend it.

Summer Milky Way in binoculars
During the summer months, point east and above to experience the richness of the Milky Way through binoculars. Even in city settings where the true outline of the Milky Way is not visible, binoculars will reveal some of that richness, exposing clusters and nebulae in the depths of the galaxy. I am particularly fond of the charts from the Great Smoky Mountain Astronomical Association for highlights in the summer galactic zone.

My old Celestron 10x50 binoculars have served me well for years and I carry them with me virtually everywhere I go. You should consider the same.

Image courtesy garyseronik.com.

14 July 2014

Mars and Spica Align

Mars and Spica Align
I've been watching Mars over the past few months as it slips steadily along the Ecliptic. It was near the bright star Spica several months ago as it was near opposition, then moved retrograde toward Porrima on the other side of the constellation Virgo, and now is back near Spica as it resumes prograde motion toward an August rendezvous with Saturn. The image shows where to spot the two bright objects in the south-west sky this week.

Image courtesy of Sky Safari.