Return to Answer

The stars.

If you picture yourself looking down at your solar system from directly above the sun, looking perpendicular to the plane of your planet's orbit, imagine that "up" in your view is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts its directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it -- all you need to measure it is a simple astrolabe, a tool that's been around on Earth since at least 150 BC and which was the basis of some incredibly accurate atronomical observations and studies in the ancient world.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our moons; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our calendar below a year).

The stars.

If you picture yourself looking at a map of your solar system showing the plane of your planet's orbit, imagine that "up" on your map is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts it directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it -- all you need to measure it is a simple astrolabe, a tool that's been around on Earth since at least 150 BC and which was the basis of some incredibly accurate atronomical observations and studies in the ancient world.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our own moon; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our own calendar).

The stars.

If you picture yourself looking down at your solar system from directly above the sun, looking perpendicular to the plane of your planet's orbit, imagine that "up" in your view is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts its directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our moons; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our calendar below a year).

The stars.

If you picture yourself looking down at your solar system from directly above the sun, looking perpendicular to the plane of your planet's orbit, imagine that "up" in your view is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts its directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it -- all you need to measure it is a simple astrolabe, a tool that's been around on Earth since at least 150 BC and which was the basis of some incredibly accurate atronomical observations and studies in the ancient world.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our moons; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our calendar below a year).

The stars.

If you picture yourself looking down at your solar system from directly above the sun, looking perpendicular to the plane of your planet's orbit, imagine that "up" in your view is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts its directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it.

The stars.

If you picture yourself looking down at your solar system from directly above the sun, looking perpendicular to the plane of your planet's orbit, imagine that "up" in your view is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts its directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our moons; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our calendar below a year).