Sky & Telescope - February 2021 - 72

Finding North by Day
Daylight polar alignment can be simple if you're prepared.

IF YOU'VE EVER TRIED to polar align
a telescope during the day, you quickly
learn how inaccurate a magnetic compass can be. Even if you correct for the
declination in your area, there's no
correcting for the rebar in your driveway,
the magnetic ﬁelds in your telescope, etc.
You're lucky to get within a couple of
degrees of celestial north.
Florida astrophotographer Alberto
Sagüés has come up with a better way.
He placed an angle bracket in a ﬁ xed,
convenient location near where he sets
up his telescope, aligned that with true
north, then uses an angle-measuring
smartphone app (AngleFinder for iOS,
but there are plenty of Android apps,
too) to transfer that heading to his
phone. The app uses the phone's internal accelerometer and gyroscope rather
than the magnetic compass to hold its
reference position, and that's accurate
to within half a degree.
When Alberto aligns his telescope,
he ﬁrst levels the mount, then he places

his phone against the angle bracket and
resets the angle-ﬁnding app to zero.
He then moves the phone to an angle
bracket on his telescope that's aligned
precisely with the scope's optical axis.
He adjusts the telescope in azimuth
until the phone app again reads 0°, and
he's aligned. With that simple alignment, he can invariably slew to see
Venus in the ﬁnder on the ﬁrst try.
But how to ﬁnd true north to align
your angle bracket? Several ways exist:
1: Do it at night, with Polaris visible.
This is surprisingly difﬁcult (especially in the Southern Hemisphere!).
But even in the north, unless you live
near the equator, Polaris is quite a ways
up in the sky. It's not precisely north
anyway, except for twice a day when
it's directly above or below the celestial
pole. You either have to wait for one of
those moments or calculate an offset.
Then hang a plumb bob over your work
surface and sight past the string, moving your head left and right until the

string is exactly north of your eye. Mark
your line of sight on the work surface,
and you've got north.
2: Survey a north line using a good
map and known landmarks. Google
Earth is pretty precise. If you can zoom
in on your observing site and on a
distant landmark that's directly north
of you (or at an angle you can measure),
you can ﬁnd north that way.
3: My personal favorite way (because
I'm a sundial nut) is to use the Sun.
At solar noon, the shadow of a vertical stick or a plumb-bob string will fall
directly north-south. The trick is to
determine when it's exactly solar noon.
The Sun moves 15° per hour in the sky,
so you don't want to be off by more
than a minute or so if you want an
accurate north line.
You might think you just need to
add an hour for daylight saving time.
Nuh-uh. It's more complicated fun
than that! You also need to add a correction for how deep you are into your
time zone. Time zones start on even
15° longitude lines. In the U.S., those
lines are 75° (Eastern), 90° (Central),
105° (Mountain), 120° (Paciﬁc), 135°
(Alaska), and 150° (Hawai'i). So if you
live in, say, Altoona, Pennsylvania, at
longitude 78.4°, you'd be 3.4° deep into
your time zone. If a time zone is 15°
wide and it takes the Sun an hour to

S Left: The bracket aligned to true north is fixed near Alberto Sagüés's observing spot, where he places his smartphone to record the direction to
within half a degree. Right: He then transfers the reading to the telescope via an angle-measuring smartphone app.

FEBRUARY 2 021 * SK Y & TELESCOPE

A LBERTO SAGÜÉS (2)

ASTRONOMER'S WORKBENCH by Jerry Oltion

Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021