How many times has it happened to you to admire the Moon in
the sky, perhaps during a beautiful clear night?
The Moon is the only natural satellite of the Earth,
from which it is on average just under 400,000 km away, or just under two light seconds: this means
that, if we could travel from the Earth to the Moon aboard the Millennium Falcon from Star Wars,
and so at the speed of light in a vacuum, equal to 300000 km / s, it would take us about
two seconds to reach our satellite! The Moon travels an elliptical orbit around the Earth,
so its distance from us is variable. If we could superimpose the Moon on the Earth,
we would find that its diameter (3470 km, ¼ of the Earth's diameter) would lie comfortably
above the United States of America, from the east coast to the west coast.
Observing the Moon, even casually, you will certainly have realized that it has a
changing aspect, evening after evening, due to the phases. But why does the Moon have phases?
They are due to the reciprocal position between the Sun, the Earth and the Moon:
since our satellite travels its orbit around our planet in about a month, the effect of this
movement is that to us, from the Earth, it seems that the Moon moves through the celestial vault:
observing the Moon carefully, we will be able to see how, evening after evening,
it rises 48 minutes later than the previous evening, and appears to retrograde towards east.
Obviously, sunlight always illuminates half of the lunar globe; but we on Earth,
due to this apparent movement of the Moon across the celestial vault, see a different portion
of the lunar surface that is illuminated by sunlight from one evening to the next.
Let's now see what are the main moon phases:
- New moon: occurs when the Moon is between the Sun and the Earth.
It is said that the Moon is in conjunction with the Sun.
Being on the opposite side with respect to the nocturnal hemisphere of the Earth, the Moon in
the new phase is not observable. This is true unless there is a total eclipse of the Sun,
which occurs when the Earth, Moon and Sun are exactly aligned on the line of the nodes; remember
that the line of nodes is the intersection between the planes of the earth and lunar orbits.
- Waxing Moon: between the phases of the full Moon and the first quarter. The Moon has the
appearance of a sickle, with the convexity of the illuminated part facing west towards the Sun,
basically towards the right, and the concavity facing east. Just after the new moon phase,
this sickle is very thin but as the days go by, the crescent moon progressively widens
up to the first quarter phase. The crescent Moon can be observed in the early part of the night.
About three days after the new moon, we can clearly see the sickle that forms the illuminated
portion of the Moon accompanied by a weak light diffused on the remaining part of the disk,
which allows us to distinguish all the rest of the disk from the sky background;
this faint glow is called ashy light, and is due to the Earth's reflection of sunlight in space.
- First quarter: occurs when the Moon is at 90 degrees with respect to the
joining line between Earth and Sun, hence the name "quadrature";
from the Earth we see that the right half of the Moon is illuminated, while the left half is
dark. It can be observed in the first part of the astronomical night after sunset; remember that the
beginning of the astronomical night by convention is fixed when the Sun is 18 ° below the horizon.
- Waxing gibbous: between the first quarter and full moon phases. The illuminated part of
the Moon takes on an oval shape and increases in brightness for the following seven or eight days,
until it reaches the full Moon phase.
- Full moon or full moon: occurs when the Earth is between the Moon and the Sun.
Being on the same side of the night hemisphere as the Earth, the full Moon has a circular shape
and is visible during all night, because its hemisphere illuminated from sunlight
faces the terrestrial night hemisphere. Since it is on the opposite side with respect to the Sun,
it is said to be in opposition to the Sun. It passes through the meridian (ie it reaches
its maximum height on the local horizon) at local midnight and sets when the Sun rises.
- Waning gibbous: between the phases of the full moon and the last quarter. The
illuminated part of the Moon again assumes an oval shape, but with the convexity facing
in the opposite direction compared to what happened for the phase of growing gibbous.
Its splendor diminishes. The waning gibbous Moon is visible in the second part of the night.
- Last quarter: like the first quarter, it occurs when the Moon is at 90 degrees with respect
to the joining line between Sun and Earth, but on the opposite side with respect to the first
quarter: therefore we from the Earth, when the Moon is in the phase of the last quarter,
this time we see that the left half of the Moon is illuminated,
while the right half is dark. Visible in the second part of the night.
- Waning Moon: between the phases of the last quarter and new Moon.
The Moon again has the appearance of a sickle, but with the convexity facing in the opposite
direction (to the east) compared to what happens for the crescent Moon.
As the days go by, the waning crescent moon tapers more and more until it disappears in
the new moon phase. The waning moon can be observed in the second part of the night.
It is good to make an important clarification: the aspect of the moon phases described here
is the one visible for the inhabitants of the northern hemisphere! In the southern hemisphere,
the phases of the moon are "the other way around", in the sense that during
the crescent moon the convexity of the sickle with the illuminated part is turned to the left,
while during the waning moon the convexity of the crescent is turned to the right.
As you get closer to the equator, however, the crescent crescent moon appears shaped as a boat,
that is, with the hump facing down, or as a bridge when the moon is waning.
The phases of the first and last quarter are also called "quadratures",
while those of new moon and full moon are also known as "sygyzies",
a term that derives from the Greek and means "union" or "conjunction".
A complete cycle of phases (for example between two new Moons) is repeated every synodic month,
that is, every 29 and a half days. Because of this periodicity, the phases and aspects of the Moon in
ancient times gave rise to the use of measuring time by months and weeks of seven days, a way of
dividing time based on the periodic and subsequent return in the course of a month of all the phases,
because the Moon, every seven days, appears with a new dress. The phases of the moon therefore
constituted the first measure of time; in the sky no other phenomenon indicates such remarkable
variations. In ancient times, the Moon phases were used for example to regulate assemblies,
sacrifices, public exercises. The Moon was counted from the day it began to be visible.
The first appearance of the crescent moon was sought with care, ascertained by the high priest
and announced with the sound of trumpets. Even today this discourse is of fundamental importance
for Muslims, because the end of the Ramadan fast is determined precisely by the moment
in which the first thin crescent moon ("hilal") appears after the new Moon.
At the moment of the first sighting, the Moon has an age given by the time (in hours
or days) between the observation and the instant of the new Moon.
Trying to identify a very young sickle has become, for many amateurs, almost a challenge. The moon of
one day (24 hours) is very dim, while a moon of 18 hours is perceptible with binoculars
but is difficult to observe with the naked eye. Descending below is a difficult undertaking.
The observation limit without the aid of instruments seems to be 14 hours.
The first observation recorded with the naked eye of a very young Moon belongs to Julius Schmidt
who in 1871 saw a sickle of 15.4 hours of age. On May 2nd, 1916, in Scarborough, Yorkshire,
two English amateurs identified a Moon of 14.5 hours without tools,
the limit lowered on May 5, 1989 in Houston, Texas, where two observation groups sighted a
13-hour and 24-minute sickle. of age. With the use of telescopes or giant binoculars,
the limit drops by two / three hours. On 7th September 2002 from Rashk Bala in Iran,
Mohsen Ghazi Mirsaeed, with giant binoculars, observed an 11 hour and 42 minute sickle,
surpassing a previous observation by James Stamm that on 20 January 1996 from Tucson,
with an 8 "telescope , he had seen a Moon of 12 hours and 7 minutes of age.
Now let's see some curiosities related to the full moon phase: have you ever wondered
what "blue moon" means? This is the second full moon that occurs in a month: usually there is
only one full moon in a month, but occasionally, when you are in a month of 30 or 31 days,
two full moons can occur; the next blue moon will occur in August 2023. Similarly, the "black moon"
is the second new moon that can happen in a month; the next black moon will occur in April 2022.
February, on the other hand,
is the only month that sometimes can host only three phases of the moon.
While the "super Moon"? It is nothing other than a full Moon that occurs at perigee,
that is, when it is at the minimum distance from the Earth.
If the Moon traveled in a perfectly circular orbit around the Earth, its distance from our
planet would be constant; it follows that also the apparent diameter of the Moon would be the same,
and therefore we would see the Moon appararent diameter is always the same.
Since in reality the Moon does not travel in a circular but elliptical orbit in a month,
it will not always be at the same distance from the Earth within a month,
but at a variable distance between a minimum value, the perigee, to about 356410 km,
and a maximum value, the apogee, at about 406740 km. As a consequence of the elliptical shape of
the lunar orbit, the apparent diameter of the Moon that we observe from the Earth will vary:
at apogee the Moon will appear slightly smaller than when it is at the perigee.
By dividing the difference between the apparent diameters of the Moon at the perigee and at
the apogee by the diameter at the perigee and multiplying by 100, we can see how the
Moon at the perigee appears approximately 12% larger than the Moon at the apogee.
With the naked eye it is difficult to grasp the difference between the apparent dimensions
of the Moon at the perigee and at the apogee, but in this case photography comes to our aid,
because it is sufficient to compare two shots of the Moon taken at these two moments.
Many think that the best time to observe the Moon is during the full Moon phase: those of you who
have tried to observe it through binoculars or a telescope will soon realize that it is not so,
because it is too bright, and the sunlight that arrives almost perpendicular to the lunar surface
prevents us from seeing its details well. In fact, the best times for observing the Moon
are around the first and last quarter phases, when the sunlight is grazing to the lunar surface,
and clearly highlights the impact craters especially at the terminator,
the border between the dark part and the illuminated one of the Moon.