Fall in the San Francisco Bay Area means clear skies and mild temperatures, and clear skies along the coast - a welcome change after the fog of summer. If you are in the Bay Area in the coming months, stop by the Ritz Carlton Hotel and Resort in Half Moon Bay for a Friday night star party. I am running star parties there a couple Fridays per month and love the setting, the reasonably dark skies, and the fun interaction with guests from all around the world. The patio is located on a stunningly beautiful stretch of California coast next to the 18th green of the Ocean Course. Hope you can make it. Click here for the schedule and more information.
This Saturday is International Observe The Moon Night. Where will you be for this special evening? I'll be conducting sidewalk astronomy in front of the Exploratorium in San Francisco along with other astronomy enthusiasts. The Moon will be a few days away from another Supermoon, and if the skies cooperate, we should have a lovely view of Earth's natural satellite in gibbous phase between first quarter and full. Click the link for more information on International Observe The Moon Night. Hope to see you at an event.
I paid a visit to the KFOG Morning Show and had a fun chat
with Renee about the Supermoon, Perseid Meteor Shower, How to Look At The Night
Sky, and Star Parties in and around San Francisco. Click here to listen.
After the Moon, the two brightest objects in the night sky
are the planets Venus and Jupiter. Venus is a close neighbor and a very
reflective planet, dominating morning and evening skies with its brilliant
white shimmer against the changing colors of the dawn or dusk sky. Jupiter is
the giant planet of the Solar System and despite its distance, is a bold and
bright object for us to enjoy, especially in a telescope or binoculars.
Venus & Jupiter Conjunction
These two planets, like all of the objects in the Solar
System, gradually change their position with respect to the background stars
from day to day. All of the objects in the Solar System move along a common
path across the sky, the Ecliptic. And from time to time these objects line up
and create beautiful patterns and visually stunning sights.
On the morning of Monday August 18th (from North
America), we will see Venus and Jupiter in a conjunction, a close alignment of
the two bodies from our Earthbound point of view. The two will be in the east
just before sunrise, so you’ll have to get up early to see this, but it will be
rewarding. The two will be less than the Moon’s width apart, and given their
bright nature, the pairing should be spectacular. Through binoculars, you will
also be able to see a lovely star cluster, the Beehive Cluster, in the
background of stars, as Jupiter and Venus will be in the constellation Cancer
and passing through the Beehive.
The image (courtesy of Sky & Telescope) shows where to
look. From San Francisco, sunrise will be at 6:30 am and the Venus-Jupiter pair
will rise at 5:00 am, so you will need a good northeastern horizon to see the
pairing, and the 30 minute window starting at 5:00 will provide the best dark-sky
viewing conditions as the glare of dawn will start to interfere by 5:30.
This year’s Perseid Meteor Shower will peak on August
11-12-13 and should offer up a moderately pleasing view of meteors but will be
impacted by the nearly Full Moon. Meteors come in all sizes and shapes and
during a reliable shower like the Perseids, you can see them all. However,
moonlight increases the ambient lighting of the entire night sky and
consequently makes the faint meteors all but invisible. The medium-strength
meteors and the fireballs will shine through the glare of course, so the
Perseids will have a showing, but just not at the rate we often see during a
truly dark sky shower.
I’ve often written that meteor showers are best viewed after
midnight, when we are turned toward the path of Earth’s orbit (we are on the
“front-face of Earth” after midnight), and we get better meteors. This still
holds true, but in a recent article in Sky & Telescope, author Alan
MacRobert suggests that early evening is a very good time to look for earth-grazers,
meteors that enter the Earth’s atmosphere as a low angle and can be seen for
much longer periods of time.I will
certainly be looking for these. I’m not an all-night observer and prefer
looking out into the sky waiting for meteors when I am a bit more awake. So the
idea of seeing grazers carries appeal for me in more ways than one. Last week on Mt. Tam we witnessed some spectacular meteors, one of which had a trajectory that suggested it was an early Perseid grazer.
For more information on the Perseids, check out these
We are in the midst of a three-month period of Supermoons, a confluence of orbital nodes that brings us the Full Moon phase at the same time as Perigee, the closest approach of the Moon to the Earth. The next one is on August 10. The difference in the Moon's distance from the Earth from Perigee to Apogee is quite substantial, varying from 222,000 miles out to 253,000 miles, leading to the a 14% difference in the apparent size of the Moon. In addition, the Moon will be at the peak of the ascending node of its orbit, placing it somewhat higher in the sky than is typical for summer Full Moons.
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.
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
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.
- - - - - - - - - -
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).
In 2005 I began writing a column for the San Francisco Waldorf School newsletter called "The Urban Astronomer." I started this blog in 2007 as a place to archive my articles and to offer additional insights on the night sky - even if you live in a big city. In 2008 I became an occasional guest on the KFOG Morning Show, and more recently on KALW and KGO. Archived shows are posted on the blog. I conduct private star parties for special events and corporate events. Contact me for details.